Thanks for this question. Obviously this is a great future direction for MIDI Designer, and in fact I wrote recently that in some future, the strict separation between design-mode and performance-mode will blur a lot.
The only way to come close to doing this now (and it's not very close), is to have two different supers for the button group. One is send-on only, and one is not.
As far as adding subs to a supercontrol, the answer is "not yet." However, it's a perfectly workable suggestion. Strangely, this is one of the features of the precursor to MIDI Designer, called Virtual Thang, as seen in this old letter.
Anyway, this will happen, but I don't want to put a timeline on it just yet.
Thanks for the kind words and the question. The best thing you can do to show support for the app is to leave an honest review in the App Store. If your honest review is five stars, that's probably best ;)