To the topic if private tuners can carry the championship - of course they can. Dakar/W2RC basically works like that. Dacia is Prodrive, which run their own team even before Renault entered, Toyota is Overdrive, Mini is X-Raid, and many of other cars without manufacturers' backing is there too. etc. etc. There always were and always will be bunch of privateers, full factory teams were really an exeption.

And as it was already mentioned in this thread, there were many attempts to build a rally car from private tuners, that ended on the need of manufacturer backed homologation.