I think it’s doable but not sure if I can build all aspects of a SCHEMA.XML file  again. Probably can’t build an ONET.XML as there are things that are created at site  creation time which you wouldn’t know about through properties and there are some things missing in the object model that would be able to extra the information.

There is already a project going on in this direction:

Daniel Larson is building on PortalTools:
http://www.portalbuilder.org/default.aspx

Did you look at that?

I also started on such a project, but for list templates... but time was
(again) my enemy.
If you are missing things in the onet.xml, maybe it is possible to start
from the onet.xml of the site  definition the site template  was based on,
and merge in only the changes?