Probably possible with a patch to sofia_reg.c lines 2414-2435, but it'll only work if you know the Realm the phone sends in advance.
A1 hash can work easily inbound since the server sets the realm and so you know it, but it'll be far more unpredictable with user endpoints. In particular it'll work when they call from one phone and fail from others when they don't use the same realm.
It'll also rely on the Sofia-SIP stack exposing the ability - looking at the API I don't see an equivalent of NUTAG_AUTH that takes a hash.