Did exactly that. Generated unlock key with kg v1.3 for FID=800, double-checked that .unl and .img have same names, turned off GPS (entered simulation mode), set a position on the map. Nuvi (it's a 1390) calculated a on-foot only route. Evidently, Nuvi does not see the MMR file. Are you sure those maps were not processed by some unlocker program (splits .img, unlocks each piece separately, then bring pieces together as unlocked .img)? 'cause once you split a CX .img you loose public transportation capability.
I also have an original CX St Petersburg 2011.10, FID=2092 (<2200, therefore, it's MMR FID needs not be modified) - works OK, calculates both on-foot only and public transportation routes, but it is not up-to-date.