I’m currently developing a complex sequencer module for the akai apc-key25. It is going very well. If you are willing to develop in c++, you can do anything you want by deriving your io structs from rack::midi::InputQueue and rack::midi::Output. As you said every button can be controlled with noteon/noteoff messages.
I plan on releasing the interface I made to make it easy to create modules specifically for this hardware (I want to make it so it supports apc-keys and mini too, but I don’t have the latter to test with rn). I’m not sure when I’ll get around to do this though…
Here’s the note mapping for the apc-keys in case someone finds it useful.
apckey25 button mapping (pad noteon messages + knob cc)
midi channels
pads 0
keys 1
PADS KNOBS
32 - 39 82 48 49 50 51
24 - 31 83 52 53 54 55
16 - 23 84
8 - 15 85
0 - 7 86 cc1/64 o- o+ 91 93
64 - 71 81 98