RainmanP

Dan,
Funny you should post this. I was browsing the QBP catalog last night and came across a clamp-on stop that made me think of your problem. The one in the QBP catalog is better looking, all smooth with a little recessed allen screw. Unfortunately, Pokey is right. These clamps are for top, seat, or down tube, sizes 1.125, 1.25, 1.375. You still need something for the chainstay to anchor that final cable housing loop to the der. You could use an Avid Rollamajig, which might be better in this case, but you still need something to direct the cable straight back.

Here is a possibility. Is there a threaded hole in the bottom of your bb shell? Even if there is not you could have your LBS drill one of appropriate size and tap it for a screw that will attach a plastic cable guide. That might solve your problem. Here is the whole picture. From your shifter a piece of cable housing down to on of the clamp on stops on your downtube, positioned about where a downtube shifter would be. Bare cable from there down to the bb, through the plastic cable guide, straight back to the Rollamajig on the rear der. Obviously you will have to check whether such a setup would work on your frame. You may have to adjust it here and there so that the cable doesn't rub the frame. One of my road bikes has this type setup except that it does have a chainstay stop so I am using a regular setup with a housing loop to the der. The plastic under-bb cable guides are in the QBP catalog, too.