Re: 72 Tele Bass Pickup
If they don't already have the parts to make that pickup, you'd be better off buying an actual old one, and then having it reworked the way you want. There's some fabrication if they haven't already done this project for someone else. You're talking only $200 to $300 for an original pickup, which you very well might be able to use as stock, without doing a thing to it. Duncan would have to, at the very least, take plain (i.e. no holes) baseplates and covers, make the holes for a bass version, and then send the cover to be plated. I'm not sure if the same bobbins can be used or not, but probably not, based on what the bobbins for the guitar version look like. Stack that on top of the normal Custom Shop cost, which is already high. And consider that they can't sell them with the Fender "F" logo (this is a plus in my book, but some people might like the F).
But the only way to know for sure is to e-mail them and find out. I'm sure they WILL do it. The question is how much it will cost.
Have you ever played one of those pickups, or do you just like the way they look? They have the same tonal problems as Gibson "Mudbuckers." This is due to both the super heavy wind and the positioning of the pickup. The cheap and effective fix for those is to wire the coils in parallel, instead of in series. That might be all the modding you need on a stock example.