I agree about the rituals, don't overlook them. Particularly from a world-building standpoint they really help to shore up the notion of the Houses. House Jorasco's success as an uberguild is going to be built on Remove Affliction rituals, not adventuring Bards.
(For many PC's, the access to rituals is going to have less impact, it's true. Reducing cost to acquire or component cost would not be out of line if you want to emphasize this aspect.)
Also, if you want to houserule a little bit, maybe there's a middle ground between just taking regular multiclass feats and the current dragonmarked feats. How about letting a dragonmarked feat also in effect be a multiclass feat that opens up Adept Power and so forth feats? So your Jorasco wizard with the dragonmark can take more feats to swap wizard powers for cleric powers. This works well for the fluff you want, since these extra powers could well be "powered" by the dragonmark and not a deity.
It would require a little thought - like, the Jorasco "multiclass" powers would have to be Healing powers, not just any cleric power. But if you're just doing it on a case-by-case basis for specific PC's and marks, rather than a full-fledged system, it seems like it would be straightforward and easy.