Another "why did I not think of that?!?"
Well, to quote the west coast hardcore punk band from the 80's DOA, "talk minus action equals ZERO!" I do come up with some great ideas from time to time, none of which are engineering or bike related, but you have to admit that my doubled sided tube of tooth paste AM/PM is a great idea (PM for night time, tryptophan or melatonin as a sleep aid and Am with caffeine for the morning brushing to get you going! But in a litigious society as this I can not risk any business venture that may risk me getting sued, so I stay poor and off the infomercial circuit)
Here is a very simple design that seems like a great modification of the downtube shifters!

I saw another great shifter "mod" that turned index road shifters into top bar mountain shifters some time ago, but can not find the link.

oh....I was never the biggest DOA fan growing up, most of my tastes stuck FAITHFULLY to the DC sounds and ideas, unless of course it came from across the pond (man I wish I had the UK SUBS Diminished responsibility on DC or THE DAMNED Machine Gun etiquette)