I must admit I haven't done very much of testing, more conceptual stuff. I tried a open source Javascript solution called HyperKeys, it does work well if you rename the windows beforehand (Titlechanger is what I used for that). But there's still some mild annoyances, mainly that it autodrags your mouse to the new window. Guess one can easily comment out some of the code...
But to be honest I won't dwelve further into all that window management stuff as I expect JMB to handle exactly those annoyances quite well / better without having 4 tools to start up and configure every time. I think the big point on all this is: splitting key down/release does work and - from my understanding it should be okayish within the TOS.
Next step for me is to wait for JMB and see how well it is handling hotkey focus. I guess we won't see instant mashing of 4 buttons, but more of a letting your four fingers slide across 4 adjacent buttons for your main AoE rotation. I'm cool with that.