Reading some older posts I found out it's a consensus that it should be more difficult to join Great Houses Inoril and Dres. Various proposals were suggested on how to implement it. Here is one of mine.
The player shouldn't be able to join them right away at all. Instead, he/she is given the opportunity to work for the House as a formal associate with the prospect to join it. If the player accepts this, he/she is included into a quasi-faction "affiliated with House Indoril/Dres". The faction should have 3 to 5 usable ranks called something like "client", "associate", "contractor", "ally", with the last one being "candidate".
As soon as the player reaches that rank, he/she is offered to join the House properly. The process itself might include some additional steps, but after it's done, the player is immediately promoted to the rank of Oathman (or its analog), since he/she has already proven him- or herself to the House.
This approach could also be used to allow players, who already joined another House to work with Great House Indoril and Dres (unless two Houses are in a direct conflict that is). Of course, in that case the player is never promoted to the rank of the candidate and can't join the House.