This may be a weird question, but I need to know if the way OpenGL aliases the edges of drawn geometry (polygons, lines, etc) is "consistant", meaning it looks the same across different platforms/hardware.
Aliasing in this case is the sharp edges that "anti-aliasing" aims to remove. I want aliasing however, and would like to use it for animation techniques that need the aliasing effect to be pixel perfect (it's for sprite-type effects). Is this feasible, or is it impossible to predict how it'll alias across different computers?
No it is not. To quote the spec (any version, Appendix A: Invariance):
The OpenGL specification is not pixel exact. It therefore does not guarantee an exact match between images produced by different GL implementations. However, the specification does specify exact matches, in some cases, for images produced by the same implementation.
So there might variation. Also consider that some graphics drivers allow the user to force antialiasing, overriding the application's settings.