I have never found the lack of a fuel guage to be too much hassle (not that I've gone truly away from civilisation on my KTMs), but I can see how they would help, or at least a guide beyond odo. Has anyone ever heard of a sight tube being added to the bottom of each (or either) tank? If just at the lower half behind tank protection, it could be safe. Otherwise a (plastic) welded section of transparrent (?) plastic to allow guestimation.
Am not sure about my 990A tank, but I think it is orange all the way through, but my 18l for the LC4E seems to be clear under the black coating, so potentially feasible to strip and clear lacquer?