I don't want to sound like the dissenting bore, here, but how can that eBay seller (listing the Acerbis taillight) claim "No changes to your bike should be required" when just about every road-bike has its rear reflector fixed into/onto the taillight-mudguard setup and this one gives no provision for one?
Having said that, I guess the XT reflector could hang beneath without looking too bodged.
I too would be interested in the implementation of this unit, Fraser and Rick. Like you, I was thinking about the feasibility of using this unit on the XT (but now I'm not so sure).
Am I alone here in actually quite liking the standard rear light and huge indicators on my 4PT? I must say that that does not extend to the mudguard, though, and have planned to chop off the excess length at some stage. The only complication is that blessed riveted reflector-mount (not insurmountable, though).