Believe it or I had considered this.. I don't think it would ever be possible to integrate ARM support into the existing UASM as the whole assembler from inception was never geared to targeting different back-ends.
I am also in the plans of making a version of UASM for 68k (specifically to support all of the new 080 features, some of which are available in VASM but not all) as well as bringing the UASM/HLL macro style flavour to 68k. I'm working with the Apollo team on the new Amiga FPGA machines, part of which include the 68080 process which is a very exciting CPU.. I always loved 68k coding and it's ISA.
For both of these projects a new assembler needs to be created from the ground-up, possibly one with a modular architecture for targeting different instruction sets and CPUs.