I got the demo ages ago and have just bought the early access, and am looking forward to playing it.

However, on firing it up, i was disappointed to find that I can't assign modal keys to mouseclicks (e.g. I can't have SHIFT+LMB, CTRL+RMB, etc., etc.).

Offering that option is not something developers do very often, but when they've done it (e.g. Cryptic in the MMO Champions) I've always found that it HUGELY opens up options for comfortable gameplay and easily tailoring things to my muscle memory, and I should think it would be ESPECIALLY helpful in translating controller-based gaming to mouse and keyboard.

I would ask the devs to look into offering the option. It won't affect anyone who's already gotten used to the system, but I think it will make the game a better m/k fighting game for new players going forward (play around with it as devs, you'll see how cool it is, and you could then offer a setup as an optional starter setup, to encourage people to look into the functionality).