We faced the same problem and we just gave up for all packages except core, though we try to be intelligent enough about it to make the use of DB specific code minimal.
After all, if someone decides to make an Oracle Version, it should be fairly simple for him/her to find -postgresql.xql files, rewrite them for Oracle and test the application.
I think a good idea would be to write a script which checks if there is always a -oracle.xql were there is a -postgres.xql. This won't help for change in the /tcl directory or changes in already ported Oracle files, but then again cvs.openacs.org and the cvs diffs would be your friend. Though again a view that shows changes on -postgresql.xql files after the last commit to the -oracle.xql version would be a tremendous help.
