Seems like that could be addressed with one of two adapters:
- A passive adapter cable with a female ATX connector (with the 3.3V and 5V pins disconnected) and male ATX12VO connector
- An active adapter cable/board with a female ATX12VO connector, the necessary circuits to step voltages down to 3.3V and 5V, and a male ATX connector.
One or both of these could happen today and immediately solve that chicken-and-egg problem for the custom market, no?