indoc week head calls were often requested by someone in the class, and then one of the candios would act all pissed off or surprised, ask if anyone else needed to go at which if you are a smart class everyone would raise their hands, then you go piss.
no one should be pissing themselves, ever. trust me you'll single yourself out more by doing that than asking for an emergency head call.
we had a guy request an emergency head call during rifle beatings... err i mean drill practice. twice, on consecutive days. the second day the DI told him to go piss in the bushes and not get caught by base police. the DI ended up having to turn away as to not let us see him laughing, and we as a class were trying our best to not laugh in formation although there was quite a bit of snickering. instant classic OCS moment.
although, asking for a head call / emergency head call during PT is not very smart at all, so piss before you head out there. you'll figure out the schedule. one tip though, get the whole class on the same page real quick that when someone needs a head call EVERYONE needs a head call, because most likely most people will and its a time away from the DI or candios inside the head. you are much less likely to get beat as a class if everyone is on the same page and raises their hand on the "who else needs a head call" question ...