Can you explain how a solution based on UTMA account is a better solution compared to a 529-based solution. 529's seem to have much better tax advantages when the primary focus is saving for college (which I assume will be the primary audience for your service).
There is also a nice price delta between the two. Stash is $9000 (currently) for 500 users/yr. GitHub Enterprise is $125,000 for 500 users/yr. That might win over some