Apologies I've been off air!!
Excellent idea Koichi
Yes it should all work. I think all you are doing is modulating the signal using IR rather that rf.
In the case of radio the GWS takes the rf modulation out leaving a signal for the decoder to work on. With IR, the sensor does the same thing, removing the 38Khz compoment.
If Jochen's experiments yeild good results then for the existing Ztron receivers you would simply need to replace the PIC and job done.
Further more you should be able to get all 8 channels
. You would need to piggy back a second PIC (and possible third) (with slightly modified code to decode the highr channel numbers).
By the way, if you are adapting existing ztron receivers, look out for the difference in PIC packaging. Most of the Ztron ones use wide (208mil) PIC the GWS decoder boards use smaller 150mil PICS.