I've seen a couple of AEGs that have been used with that Blammo Marker stuff.
It does work as advertised...leaves a mark on what it hits.
It does leave specs (looks like dust) on the inside of the AEG as well. Didn't seem to "build up" or clump...just like a dusting.
On one PTW that had been used with it...there was red dust everywhere (don't know if it had chopped the marker bbs or not). While the function of the rifle was NOT impeded in any way...it was everywhere. Aside from the visual aspect I didn't see any negative affect (i.e. didn't cause mis-feeds, didn't block the sensors, gum up the gears, etc..). I would highly suspect that the dust would be/get on the hopup rubber...and I can't imagine that anything that would reduce the "grip" of the rubber would be a good thing in general.
Neat...but I wouldn't use them by choice.