I don't know why it didn't occur to me sooner, but the most Rails-consistent way for ActionHelper to support a controller declaring action-specific helper modules is pretty obviously extending the existing
ActionController::Helpers#helper method to accept an options hash supporting
:except, the same way the filter class methods do.
It may not be the coolest way to do it, but it's definitely the least surprising.
So ActionHelper now supports
:only and will likely support
:except sometime soon. (Though except seems much less likely to be useful.)