I'd have no fear of problems arising from welding that classic plastic vs alloy groove. It's away from the main stress points within an AF frame, other than the down tube / Y - that is at the points where the spar is welded to the shock tower, the cracks propagate from the inside , then the top , bottom and eventually the outside of the spars with the natural flexing of the spars - your groove is about 80/90mm away, by the looks of the fuzzy picture.
A nice bead of 5039 - best for what I believe is 7000 series alloy, but 5356 is perfectly acceptable ( I use 5356 pretty much exclusively, as it is suitable for all the grades I deal with), by a competent TIG welder, then your choice of finishing tool and off you go.
A little bit of clear contact at touch points goes a long way - I've seen frames , and more often, triple clamps / forks worn all the way through by plastic / cables rubbing against the soft alloy.
I'm using Gen3 shrouds on my alloy tanks, and any edge contact point has a small bit of clear contact on the tank.
First thing I did on my new 01 CR250 (When it was a 250), I put a couple of pieces of 3M graphic material in that same spot. Its still there now - with no issues. I wonder if you could weld on it and blend it in...
The CR500 is an acquired taste. If you don't like it, acquire some taste...