I'm sure someone far more knowledgable will answer this concisely very soon, but for what it's worth here is my take on this...
Funnily enough I've been looking in to this conversion for my A2 project and, as per usual, have come up with many conflicting posts! Caliper wise I guess it all comes down to the bolt pattern of the caliper to hub/upright and at further thinking the offset that the caliper runs vs the hub & disk...
I have read that to run the LCR's on my A2 I'd need to use TT225 hubs only, however I am still not 100% on that information as people also state that if you're running the 288mm brakes, as per Golf & A3 1.8T & Leon Cupra you can just bolt the 312mm, TT225/S3, carriers straight on... that leads me to belive the carrier bolt pattern must be the same on the hubs between the Golf/A3 1.8t and the TT225/S3. Further more the LCR brembos are direct fit to the S3, so again, in theory, the LCR Brembos must use the same carrier bolt pattern as a Golf/A3 1.8T (or am I missing something?!!)
Where am I going with this you ask?!!
As luck would have it I have the stock 288mm set-up from my A4 kicking around and I also have an A3 1.8T donor car so I'd be more than happy to pop outside for you to see if the A4 caliper fits on to the A3 hub and over the A3's brake disc. Surely that will give a good indication as to whether LCR brakes fitting an A4 has mileage in it? Oh and that is with me guessing you're using the original B5 uprights and not the S4 ones?
I do admit though, unless you're getting a cracking deal, I must ask if it is worth all of the hassle? LCR brakes seem to command a huge sum of money for what they are in my opinion and then you'll have to find discs or re-drill the 323mm LCR discs to x112 stud pattern anyway. Surely you're getting close to hitting RS3 brembo prices by then? But that's another post entirely I guess!!
Anyway I hope this helps and sorry it is so long-winded!