Never heard of DWPs being issued to non-residents, but I've also never had a need to look that up.
Normal DWPs are very hard for anyone to get, including the DD DWP. You pretty much have to be an SOT and have some sort of related business the State considers legitimate, like film rentals, training LE or military, sales of such weapons to LE or military (or manufacture for that purpose), etc.
However, unlike for other DW categories, there is a collector's version of the DD DWP that is attainable by ordinary people. Besides passing the background check and showing that it's for collector purposes, you also have to meet some secure storage standards which can be a bit much for some (such as apartment-dwellers or those with limited wealth/income) and agree not to discharge the weapons in CA and to keep a log of public displays of the weapons. Also, DDs under .60-cal. do not need a permit.
I forget, if an NFA weapon is C&R, does that mean that it can be transferred to a C&R FFL holder without going through an SOT? Seems like the transfer would be good in that case Federally, but I could see there being State issues. Just because a transaction is valid Federally does not mean it is valid in CA. Possession without appropriate permits would be an issue, too, unless there is some exemption of which I'm unaware (certainly possible). Is there an issue Federally having it shipped to you? Shouldn't be a State issue and if it's GTG that way, that might just be the safest way to go about it, even if it is less convenient.