multi axis toolpaths on stl files?

so i'd like to start by crying a little bit about this stupid file. stl files suck. especially this one. it's huge, and i mean huge. it's got to be like 3trillion lines or so. (ok maybe a bit of an exaggeration, but man, not that much)

anyway
what i'm working on is a head. yeah a 3-d head on a post.

i believe that there is an art add-on for mastercam which is what you're supposed to use to make sculpture but i am without such niceties. i do however have a nice five axis version X4, but i cannot get the multiaxis tool paths to work because i can't select the stl mesh as a surface for the purposes of these tool paths.
with the three axis 3-d tool paths i can select the entire mesh and use a chain to limit the machining areas with no problem.

ideally what i'd be looking for is a 4axis toolpath so i can just rotate the part about an axis and cut my head out.

so if someone would be kind enough to point out the simple solution i just totally missed (fingers crossed and holding my breath for this one), or maybe suggest a cheat i would appreciate it a lot.

thanks for taking the time.
Rob