I had an old mobylette when I was 14 or so and I put some custom french headlight on it that had a "tell tale" red light......but the reflector insidewas not the original and no light came to the "tell tale". So I spent a few minutes with a dremel type grinder and hey presto....it worked great.
The key to insuring that removing a section of the reflector doesn't bugger up your headlight beam is to have the tell tale and thusly the ground reflector point close to the rim of the reflector......you could not see the ground area on the one I did.
If you grind any portion further than about 2 inches back your gonna' see it.
I also reckon there is no reason one couldn't use those red dash warning light lenses and bezels and grind the bezel under it....should work fine.
If those lenses were too flush to the lamp body then grind an angular facet into the rear face of the lenses and it would work for sure.....
Just my two cents....
By the way Old Nail was just commenting on tell tale lights for his beauty.....it must be something in the air.
says that: 
or it's just too high tech 