OK, so, at first I tried it exactly like that and the script seemed to work until I tried right clicking, which didn't work.
I also tried including Rbutton::Rbutton before Return and it only got worse.
Also, to get the full picture, I tested this in a separate script, but my goal is to integrate this piece into my rather complex script file that contains all the scripts I use, and I wouldn't want this to cause any issues.
As a detail, I use Win XP x86, but that shouldn't be a problem.
After further tests, I figure out that I got the best result with this:
~RButton & LButton::
With some research, this works for allowing normal use of the Rbutton, as well as the combination R+L, but it can only send the "a" key and not the click afterwards (tried various methods, mouseclick, send lbutton, etc.).
It also right clicks at the beginning, but for my purposes that's a bit of an annoyance that I can live with if the rest of the script works as planned.