I somehow managed to forget to put in the section on drawbacks, and solutions to using a 'caliphate' of Orsinium, so I'll add that here.
Most relate to it being tied to religion.
First, this would make Orsinium a head of religion title. For a hypothetical Trinimac religion, there isn't really any issue with this given that Trinimac is pretty much just an Orc god, but for Malacath there are non-Orc worshippers. Personally, I'd be inclined to suggest that this isn't really an issue - most Malacath worshippers in the game are Orcs, Orsinium is referenced as being home to all sorts of Malacath creatures (at least in some of its incarnations) and Malacath religion is already geared for Orcs anyway. Still, others might find that more of an issue so its worth considering.
Second is that the 'caliphate' would be unavailable to non-Orc religions. Simply put I don't really see this as much of a problem; Orsinium is very much a cultural centre for the Orcs, so a Warlord who doesn't have the support of many Orcs is going to have a much harder time establishing Orsinium than a less powerful warlord who is better liked by the Orcs as a whole. If such a 'caliphate' system was implemented alongside province modifiers and/or special buildings for Orsinium then what could happen is that upon establishing the 'caliphate' you establish Orsinium (or vice versa) and if, upon establishing the kingdom title, you didn't already have those bonuses you would get them. Thus correct religion Orcs have a relatively easy time establishing Orsinium, and wrong religion Orc can still get establish it, but have a harder time doing so, requiring the kingdom title - I think it would be reasonable to assume that while Orcs would be unlikely to raly behind a relatively minor warlord of the wrong religion, when other options are available and creating Orsinium is an uphill struggle, they would be more likely to accept a King of wrong religion who had already proven himself capabe of establishing a homeland for the Orcs.
Third issue, though not exclusive to using religious titles for Orsinium, but which does affect them and still needs addressing, is of the possibility, which AFAIK is never explored in the canon but perfectly possible in CK2 is that Orsinium exists in some capacity, but is not the most powerful Orcish state. I woud simply give all Orcs a CB which takes whatever province is designated as Orsinium, and either takes or destroys any 'caliph' title depending on religion. Indeed, although I forgot to mentioned earlier, the 'Caliphate' should be entirely dependent on having Orsinium in the realm with it being immediately destroyed on its loss, and immediately created or usurped upon capture of the province designated as orsinium.
As a final idea, destruction of orsinium. If no liege of the province designated as Orsinium is Orcish there would be a timer (50 year perhaps?) after which the province loses its status as Orsinium, representing either the Orcs losing interest in reclaiming the sacked land instead of just rebuilding somewhere or, in the case of revocation rather than conuest, it losing its status of importance to Orcs. During this time, Orcs get a CB to retake Orsinium, and if they fail to do so, they can rebuild it somewhere else. Similarly, if there was no province marked s Orsinium, and the Kingdom existed dejure, but had no holder, its land could revert, based on region, to its original provinces.
As a final final idea, such 'caliph' could be called 'Orc-Lord', and if I'd thought of that sooner there wouldn't have been so many quotation marks everyehwere.