Never say never, but CO VERY rarely overbooks BF, so IDBs are extremely rare. I have been able to volunteer a F seat on a VDB domestically, but have not ever tried from a BF seat internationally.

Edit to add- I completely misread and thought you were asking about bumps from BF, not to BF. I've never been given the option on a VDB to also get BF on the new flight, and I've asked. If there are empty BF seats I'd think that CO would OpUp someone rather than IDB, but you never know.