It may depend on which version of the P229 you are considering. There are plenty on the approved list, but I'm not sure that all variations of the P229 are.
From the CA DOJ Roster of Handguns Certified for Sale:
Also, regarding your .357 Sig barrel swap idea, I have read elsewhere that the sights are different for slides that came on guns with .40 barrels than those with .357 barrels. So after doing the swap you may have to adjust your sight picture. . .