OK, any 88-note keyboard will give you 88 notes to choose from. That's a bit. And yes, I know there are 128-notes(0 through 127 = 128) to select from. Accesssing those "unreachables" can be difficult, but not impossible.
Now, 300 possible scenese seems like a lot of work, but that's really not a big deal.
Here is my recommendation: MyDMX doesn't care WHERE the MIDI trigger is coming from, just rather what is the trigger. Of course note on events are the easiest to control, hit a key and boom, there it is. MyDMX doesn't care what MIDI channel it is, so you can easily obtain the triggers you want, but you're going to have to burn additional MIDI channels to do it. It does not appear that MyDMX is saving velocity, aftertouch and note-off data, but is keying off that note on for sure.
If you want to get super picky, each MIDI channel has 128 MIDI notes, each with a potential 128 levels just for velocity, another 128 for aftertouch, and another 128 for note off. There is also the capability of 128 total things per channel AND per note, so really, technically there is a LOT that could be done.
Personally speaking, note on is the easiest way to trigger.
My advise would be to do some paperwork. You've made your scenes, now assign a note value to them ON PAPER first. This is so you can sequence or trigger it later.
Going old-school, using MasterTracksPro on the Mac, I had relatively easy access to 128 notes on any channel via the on-screen keyboard. What I am saying is you might have to think outside the box to get all your programming done. An 88-note keyboard may be able to be adjusted up or down the notes(my DS-8 can transpose), or else perhaps augmenting with percussion triggers to a brain to hit those other values. However, I bet most software sequences let you use the moues to input data, so I'm thinking that would be the most cost effective way to go.
However, this is my advise:
If you're going to go "MyDMX and sequencing, the best solution is 2-computer solution: one as the sequencer, one as MyDMX. ALSO, you NEED to use a multi-port MIDI interface, such as the M-Audio MIDISPort 2X2(for example). The reason for this is that you're going to need to tie the machines together via a MIDI cable, assign all your DMX to a port(say, B for example, using A for music).
This CAN be done as one-PC solution, but you have to ensure you're not looping MIDI as that will cause a MIDI feedback loop and cause your computer to hang. You also need to give yourself enough lead in to switch to MyDMX so it can be the foreground application. You run into a greater chance of crashing and problems doing it as a one-computer solution, and if you're on stage, you don't want that.
In closing, you CAN do what you want, but regardless, you'll need at least 3 MIDI channels, assuming 128-notes each. If you feel better only using 88 notes per channel, then you need to dedicate 5 MIDI channels.
If you're feeling especially clever, you COULD bury your DMX data in your regular MIDI sequence using unused note values, which is fine if your equipment supports "restricted ranges of notes", which most modern stuff does support.I personally wouldnt' do that, but it COULD be done.
I don't recommend continuous controller devices, such as knobs and faders. While MyDMX will respond to it, it's not as reliable.