I was actually wondering the same thing. I started off in engineering as an undergrad and got lazy with it so I switched over to a liberal arts type major.
Would love to go back and finish what I started somehow.
You want to get a M.S. in engineering with a liberal arts degree or want to go back and take whatever classes you didn't finish to get a bachelors? Which one?
Getting a masters without an bachelors in engineering/math/physics etc is going to be near impossible. Most graduate programs REQUIRE an undergraduate degree, most require a degree in the specific field; i.e. if you want to get a M.S. in aero, you need an undergrad in aer/mech. Maybe Devry has something. Or that online University of Phoenix.
Here's some admission criteria from my alma mater:
-Bachelor of Science in engineering or science (including sufficiently quantitative social science programs) from a regionally-accredited institution*
-Two years of calculus through differential equations, or an equivalent intensive one-year sequence
-Cumulative undergraduate GPA of at least 3.2 on a 4.0 scale
-One course in engineering economy or the equivalent material in a combination of other courses (M.S.ISE, M.S.EMT)
-One course in linear algebra (suggested)
I doubt you took linear algebra for a liberal arts degree.
Going back and completing the classes necessary to get your bachelors... I'd say you would have to go back to the same school and program you ditched. Plus, curriculums change fairly often so you might end up having to repeat nearly everything minus the basics (physics/calc/chem). Going to a different school and saying, "Hey, I took phys/calc/thermo/statics at another school, can I finish up here?" would most likely get you the response of "get the hell out."
So I'd say you're screwed in either case.
On a side note, I'd be pretty pissed if I went back to school for an engineering masters and there was some guy with an undergraduate liberal arts degree is sitting next to me.