As far as I know, the basic assistance device for chins is resistance bands. These have the drawback of helping you out of the bottom but providing nothing at the top. All advice I've seen on improving chins seems to hinge on the fact that there is no device that provides constant assistance through the full range of motion (well maybe at a gym, but not at home).
So first question: is there an affordable home device for doing this I've never heard of?
Assuming no, I was pondering how to make one, and it seems simple. Take a 10 foot 2x10 plank. Put a groove in it about 2mm deep and 2mm wide at dead center. Build a little fulcrum about 1' high, drop the board's groove into the fulcrum. This gives you a teeter-totter. Stick a length of pipe on one side (standing vertical) for loading plates and you just stand on the other side. Load a 45 lb plate and you are magically 45 lbs lighter. Probably can't get cheaper or easier than that, assuming there is someplace you can slide a 10' plank so it's out of the way. Only thing I figure might be a problem is that the board is never really flat, so your feet cannot be flat on it. If there is no cheap device out there to buy, I might just build it and see how it works.
This would also be a cool way to lighten the load on a deload day for dips.
Sooner or later of course "deload" for me will be bodyweight on these exercises, but methinks that will not be for well over a year.