Sure, yes you could do that -- I mean, PrivateStorage is just shipping you a "real actual Tahoe client". The main feature you're getting is the managed storage-servers.
So if you happened to "not completely trust" the availability of those you could also configure one of your own and configure your client(s) to use that and the PrivateStorage servers. That is, hedging against PrivateStorage going away so suddenly you can't retrieve your data.
But, I agree: if you're doing that you're likely able to run your own Tahoe grid on VPSes or similar.