I belive it's this you were talking about:
http://www.mmmonkey.co.uk/nes-to-pc-engine-controller-mod/unfortunately, I don't think that would work...
I do have one idea though.
add a port to the NES controller (DB15, perhaps?) and then gut the TG controller and wire the traces to another jack, then use a cable to connect them?
a bit of a frankenstein solution, but I think it would work, the NES pad is easy enough to map out, and though it's been FOREVER since i've opened up a turbob pad, I think it wouldn't be too hard to find decent solder points.
the one downside i can see to this is maybe losing the turbo on the turbob controller.