Is it also possible to allow the “Designate Heir Title to convert that title and associated counties too a Viceroy type title. That way if your heir dies the title reverts back too you and his family rejoins your court. That way you can arrange things as you want them dynastically.
I like the idea. Nevertheless, I might be mistaken, but I think you are talking about "Appointment Succession" instead of Viceroyalty. Upon death, a vicereine would revert its higher titles (duchy or kingdom level) to the crown, but any county level title is passed ahead normaly to the previous onwer heirs, while Appointment is how the Vale's Bloody Gate county tier title work, for instance, in which there is no direct inheritance of the lower level titles upon the death of the current holder and all titles there revert to the crown at once and the family of the previous holder moves to the capital. Again, I might be wrong.
But if game mechanics allow it to be created by decision, I would certainly love to have the "Designate Heir Title" to create one of those Appointment Only Succession cases. But how would that be reversible when another dynasty takes the Kingdom? After all, supose previous house sets a landlocked county in the middle of nowhere as his official Designated Heir Title and I, as a Celtigar take over the throne and want to make Claw Isle mine?