For personal time-related reasons I have dropped out of participating in working on the OSC altho i think it is a great project. And also because i realise that it isn't so suitable for my purposes.

However i would be happy to test out categories that the working group comes up with. But that would require another field, i think. (An alternate would be to use one of the existing 2 classification fields -- DDC, LC -- for other than the designated classifications for those fields, but i don't think that is such a useful idea, particularly if someone wanted to do some kind of comparative testing.

I guess this question goes to Tim. How possible would it be to add another sortable field?

Yes, please!
I have followed the discussion about OSC for a while, but as the project didn't seem to progress I've lost touch.
For me it would become much more interesting if I was able to add the OSC classification to a work (or edition, or item).