The ply format looks quite simple and you should be able to get something going quite quickly as a plugin. Look at the sv_* plugins in the SDK for examples.
Some of the example plugins need updating for 4.0 - I'll post a migration message soon.
The AC3D file format doc is
http://www.ac3d.org/ac3d/man/ac3dfileformat.html
Writing import/export plugins is easy with AC3D - but do post any questions you have here.
Andy