You mention twice that you need to charge clients by the hour, but why do you feel that this is necessary? Right now your clients pay you a percentage of their assets via trailer fees, what prevents you from charging a similar percentage on top of what the mutual fund expenses, which (without trailer fee) are now cheaper to begin with?
This is more or less exactly the current model - acquire clients with low net worth by charging by percentage of assets (which isn't much for low-net-worth clients) and retain them until they're wealthy enough to pay off. The only major difference is that billing is more transparent and clients now have a choice between being charged by the hour and being charged by percentage of assets on top of the same lower-fee mutual funds.