Do you have something to cite for that? I know of the critical and essential exemption but have never heard it used as a blanket exemption for 1811's and I don't think it would be legal. I wouldn't be surprised if some agencies might try to do that for their agents, up until ~10-15 years ago at least the FBI and Secret Service prohibited their agents from being in the reserves, but that was found to be illegal and so I imagine an exemption from MOB's would be as well. No one, not even federal LEO's, is irreplaceable.
Yes: 32 CFR Appendix A to Part 44.
The federal law is VERY explicit, specifically detailing:
"The Deputy Secretary of Defense shall adjudicate, before mobilization, conflicts between the mobilization manpower needs of the civilian sector and the military that the Ready Reserve screening process has identified, but has not resolved.
To ensure that Federal employees essential to the continuity of the Federal Government are not retained as members of the Ready Reserve, the following guidance is provided:
Conduct annual screening program as provided for by the Assistant Secretary of Defense for Reserve Affairs.
Federal Agency heads, or their designees, concerned shall designate those positions that are of essential nature to, and within, the organization as “key positions,” and shall require that they shall NOT be filled by Ready Reservists to preclude such positions from being vacated during a mobilization. Upon request from Federal Agencies, Secretaries of the Military Departments shall verify the essential nature of the positions being designated as “key,” and shall transfer Ready Reservists occupying key positions to the Standby Reserve or the Retired Reserve or shall discharge them, as applicable, under 10 U.S.C. 10149, except as specified in § 44.4 (b)."
It is a black and white issue under federal law -- emergency essential key government personnel may not be legally permitted to serve in the SELRES. Of course, there are a multitude of 1811s and others who fit into this category serving SELRES in the Navy Reserves, but it must be understood they are doing so at their own risk and more often than not, simply not declaring their status as a key government employee to the Navy Reserves, either in an attempt to not be transferred to the IRR or because of their own ignorance (I know many 1811s, particularly from non-premier agencies, who have no idea about this and thus always check "no" each year during the annual screening).
FBI, USSS, and others are all still emergency essential "key" government employees and their civilian agencies are fully permitted under federal law from prohibiting them from mobilizing, even in the event of a volunteer mob. Internal policies within FBI and USSS prohibited their SAs from serving as reservists historically, however these were merely internal agency policies; the internal policies now permit SAs to serve as reservists, however their status as emergency essential key government employees at no point has changed and both agencies, in accordance with federal law, can reject any mobilization they military may attempt to levy whether voluntary or involuntary. All military branches have chosen to approach the law differently; for example, the Army Reserves will openly allow emergency essential key government employees to serve in their selected reserves and will not involuntarily mobilize these individuals without pre-coordination with their civilian agencies. This, of course, requires significant manpower to enable the flexibility that the Army Reserves has chosen to apply so as to ensure these individuals can remain SELRES. On the other hand, the Navy Reserves has decided to not waste their time on anyone whom the Navy cannot involuntarily mobilize at any date, time, or place of the Navy's choosing and the Navy's internal policy is to swiftly transfer, pending review at Millington, ANY Navy SELRES reservist straight to the IRR upon receiving an annual reserve screening questionnaire indicating their status as an emergency essential key government employee -- this can be verified with Millington.
As a matter of policy and consistency, OPM has designated the 1811 designator as an emergency essential key government employee regardless of agency, and federal agencies, particularly the smaller ones, simply defer to OPM policy regarding these matters. The OPM policy is for blanketing emergency essential "key" coverage of 1811s is for a variety of reasons, but the gist of it is that for the same reasons 1811s and only 1811s receive LEAP, these are the same reasons for which they are blanketed emergency essential.
Under the scope of federal law and OPM policy, the clear interpretation can be made that FLEOs holding the 1811 designator who are emergency essential key government employees are irreplaceable over military duty, however the boat load of 1801s, 0082s, and other federal LEOs are not (unless their specific agency designates them as such). I know many 0082 DUSMs who do 10x the work of an 1811 bureacrat, but the law is pretty clear.
For anyone who does in fact find themselves in this dilemma or is reading this thread and now questioning their own status, the most responsible thing which I would strongly recommend would be to consult with a federal labor attorney or your unit JAG before filling out your next annual reserve screening questionnaire to ensure you do the right thing as required under federal law and do not find yourself in a bind later on when the Navy has involuntary mob orders with your name on it -- that is not the time to realize your status as an emergency essential key government employee.