Just curious, are you trying to generate a plane constraint for export reasons, or to assist in modeling?
Something else that might (or might not) be helpful, the "Move Toward" plug-in. It has source code, too:
http://supercoldmilk.com/ac3dplug/movetoward.html
If you are only looking for the constraint for modeling (won't help if you're writing an exporter) this trick sometimes works: AC3D will constrain to the major axis if you hold down the control key. If you have a plane that's on an odd angle, one thing you can do is rotate the entire model so that the plane you want to constrain to is aligned to an axis, then move your object using the axis constraint with the control key, then rotate the whole thing back to it's original position. This trick works well for planar-mapping sloped objects, too.