I think they already coved that he can't be decrypted... I do however believe that we will, in the next book find a whole bunch of new allies for Parson, hopefully one with the ability to revive him (white magic-type stuff) and not just make him extremely undead.
Wanda + pliers is the most powerful croakamancer in Erf, and she cannot raise Bogroll. I think it would be a pretty large deus ex machina if raising is possible for other "basic" casters. If raise dead is possible, then why was Ansom so surprised.
One possible way would be if some other caster can reverse the burning step, and effectively produce a dead body from the ash of its remains, and then Wanda could decrypt. Ofc, for that to work, someone would have to 'claim' the ask so that it doesn't depop.