Not sure if you can do it in Reaper or not, but AutoHotKey should do it with "WinGet" so if the current window = Midi Editor it will send right click when you double left click. You may have to change the Mouse Modifier Default Double click to do nothing rather than delete note.
BUT, you need Windows for AHK though there are Mac modifiers and apple script that may do the job.
EDIT: something like this
WinGetClass, class, A
if %class%" = REAPERmidieditorwnd then
See if you find a solution here if not and you want to try AHK give it a go, I haven't AHK for a while but I can look into it if you get stuck.
I added it into my mouse screen zoom AHK: