Yup, you should be able to with a C&R 870 SBS. The issue becomes finding a mount for the 870. There are apparently a couple designs out there that go in an out of production.
Just make sure that whatever mount you use does not let the rear of the 870 come in contact with the front of the mag well under recoil. Bad Things can happen if that occurs. Theres also a mount that hangs off the front takedown pin that transfers the SBS recoil into the lower receiver and may cause problems.
As for CA NFA dealers willing to do work with us, haven't found anybody except for M24 up in NorCal. I still need to talk to my local guy but since I'm not ready to spend more money, I've put it off. Don't want to look like one of those looky-loos.
There is one other possibility that I just realized, an FFL doesn't have to be an SOT to transfer an NFA firearm. Any FFL can do the transfer, they just don't get FFL-to-FFL transfers tax free. Now, for an SBS, that adds a second $200 tax that makes it unfeasable, but for an AOW, its just an extra $5 tax stamp. That might be doable and would open up the dealer search to any FFL willing.
Do you want an AOW or C&R SBS/SBR in CA?
FrontSight Training Course certificates available $25, PM for details on them and other options.
No posts of mine are to be construed as legal advice, which can only be given by a lawyer.