If it were to work, then it should have long-term focus. So act as a busines angel/VC "knowledge fund". Get a few percentages of ownership and hope to one day have advised some successful companies. Maybe have a deal where you transition from On Demand CTO to Board member as the company grows and can hire a fulltime CTO.
Another point should probably be to only take on startups that have actually been incorporated in some way that enables percentages to be exchanged. Otherwise I can see a lot of wasted time on peoples great ideas.
Seen from the other perspective you would probably also struggle a bit to gain traction. Have a good track-record for founders to contact you and give a way a few percentages. They should be comfortable that you can actually help them and that they can have enough of your time to make a difference.
After writing this I actually think it less of an un-viable idea than when I started writing :)