Hey JonL,
I did this same thing last summer. I wanted to add turn signal capability to my Jeep but keep my original single filament front lamps. I found that the older 800 series Signal Stats did have allow turn signal and parking lamp functionality with single filament lamps, but they're long out of production. So that leaves us with the 900 series Signal Stats, but they require dual filament lamps.
Soooooo....
I took my 900 apart and noticed that the buss bars inside the unit could be modified to allow exactly what we want: the ability to pass parking lamp power to the single filament front lamps, or inject the turn signal power. It's essentially the same thing the REAR tail lamp/parking lamp does...single filament used for both parking and turn signalling. The mods I performed involved cutting a bar, creating some new contacts with some brass screws(the sockets are empty in there ready to use!) and some soldering inside the unit. The mods are highlighted on my "1950 CJ3A nuthin' fancy" thread, post #86. in the "Your Stock 3A Project" section of this forum.
http://www.cj3apage.com/cgi-bin/3Ayabb26/YaBB.pl?num=1414467408/75#75For me, the mods were simple, and worked perfectly, but I can see where alot of folks wouldn't want to do this...lots of small detail work...I'm thinking there may be a market. You could probably also sell a bunch at one time to some of the Jeep suppliers, although they'll definitely want 'em at low cost! Good luck!