Lotek, you are correct, some of the Modolo levers provided the option of internal or external cable routing. It's been a while since I've done one, but I believe you remove the carrier/cam that holds the cable end. It should be held in place by a Allen key nut and bolt that goes though the lever body. The carrier/cam is replaced by a nylon pulley. The cable is then routed down through the top of the lever body (the normal cable exit hole) so that the cable end plug sits in recessed hole where the cable housing end normally seats. Route the cable down past the pulley and then through the back, bottom of the lever body, where there should be an exit hole.

I'm not sure if this is 100% correct, as I'm going solely based on memory. Hopefully, someone else can corroborate this. If not, I'll check through my literature on the weekend. I believe Modolo had an ad in the mid 1980s that showed cross-sectioned levers with both routing options.