I think this is a worthy proposal and it does indeed address a salient problem. Initially, I wanted to postpone its discussion until the named topic support is implemented; however, @maksim.drachov is unable to continue his work on the named topics support due to other commitments at Zubax, so delaying this further is probably unwise.
A related problem was brought up just recently at Subject-ID Assignment.
I think this option is the most aligned with the existing design. We can modify the uavcan.register.Access specification accordingly to permit arrays and the bracket syntax. The same specification should also provide a detailed explanation of the feature (close to what is written in the OP post).
I think there is merit in extending this proposal not only to subscribers but also to publishers, as nodes that publish multiple related topics are not entirely uncommon (continuing the topic of ESCs, it might be a 4-in-1 motor controller). Another matter is the inner consistency of the specification: publishers and subscribers are two sides of the same thing, and any significant divergence between the two is undesirable.
As there are no other comments so far, I assume there are no strong objections to the proposal, hence I recommend @Dima to submit a pull request to the standard DSDL definitions so that we can discuss the specifics further.