I am not aware of any real tooling support for setting start levels for bundles on the feature level. The only option you have is manual hacking with a p2.inf.
I think the reason that setting start levels is only really supported for products is that p2 can't handle the case where start levels are specified multiple times. This could easily happen if setting start levels on feature level was encouraged.
So, you can make this work on feature level, but only if you know what you do.