wow, loadsa questions!
To answer a few in one go; SPDIF has the clock source embedded in the signal which means ANY SPDIF signal is trying to be clock 'master'. This is also true of AES/EBU (but the electrical properties are different as is some 'header' information) and ADAT which is completely different. So, with this in mind:
What does the S/PDIF input on the GT expect in terms in clocking ? - None, the GT will lock in on the incoming data stream.
Does it expect that input signal is synced to the clock from the GT's S/PDIF output ? - No, the two operate indepently.
Or does the GT sync to the incoming S/PDIF clock when in sampling mode ? - Yes.
I am guessing the latter, which is presumably why you can't use it for the Audio Input oscillator type. - strange, I have an XT and when I get some time, I'll look into this, but this should be OK.
Of course I'd much rather the GT received an OS update that allowed it to sync it's S/PDIF out to the clock received on it's S/PDIF in. - Impossible because it's limited by hardware, it's just not the way it works.
ALL SPDIF equipped gear behaves the same way unless the instrument is capable of syncing to an external master clock, in which case, that 'master' frequency should be reproduced at the digital outputs.
Right, on to your actual problem solving magic box -
"Ideally what I'd like this magic box to do for me is take the S/PDIF output (coaxial or optical) from the GT, reclock it to an incoming wordclock (BNC) and then output the reclocked S/PDIF stream to my audio interface (coaxial or optical). Since the output of the GT can be switch between 44.1/48/96k I wouldn't need the box to resample."
In this description, there is actually no need to resync the SPDIF output. You could connect the GT straight to your audio interface. Why do you want to resync? The only time you need to resync digital signals is when mixing multiple channels to then run on one master clock.
For example, say you have two synths with SPDIF outputs which needed to be mixed together (just as analogue), in this case, both signals need to be mixed to one 'master' clock. This can then be converted to say another digital format or to analogue.
I have a very complex digital setup where I have four SPDIF equipped synths - input and output, which go from SPDIF on RCA plugs to AES/EBU on XLR and then converted to ADAT lightpipe into my Akai DPS24. And then I can go all the way back again using the Akai as a digital patchbay. This means I can transfer audio from ANY source to ANY destination all in digital.
Give me a precise run down on how you see your setup working and we can talk further.
Steve