Always late to the party....
I only have one suggestion (from a total newb)....
What about putting an exacto blade into a toolpost somehow? They seem pretty rigid/stiff to me? Likely have to hold it so it cuts with the top of the blade (ie. the part opposite of the actual cutting edge), but I assume this would give a pretty thin groove? It should work on alum, no?? Or, perhaps a broken exacto blade shaped in such a way to work like a mini-cutoff tool?
Edit: perhaps there's even a way to stack several of them together (with spacers) so that you cut several grooves at once??
Anticipating laughter and mocking.... but a guys gotta learn somehow...
Murray