Why not just allow complete customization? Let the sponsor chose how much of what they want to pay from the war chest. It would certainly make some armies more desirable than others. Besides, this is a broad stroke thing. If the sponsor wants to pay fro 100% of recruitment, they will be paying for the neighboring dukes SF's if he is in the army as well as the new knights awesome archers.
Recruitment: 0-100%
Training: 0-100%
Repairs: 0-100%
Travel: 0-100%
The problem with allowing this is that abuses are very easy.
Realms could simply funnel all of their realm gold into the hands of one person and have him cover 100% of all expenses at all times. That leaves much to be desired about the system if someone chooses to do so.
