Thanks for your question and your curiosity. MIDI Designer doesn't provide a way, currently, to stack up notes and send them out later (on a button press, for instance). We'll consider this feature for the future.
Thanks for taking the time to explain what it is you're trying to do. It's definitely an interesting feature and it will eventually work its way into the App. Hopefully it'll be in a way that is compatible with the hardware you're trying to use.