Issue looks to be resolved. Have tested new mutter package and verified i'm able to map buttons on the cintiq, so this can be closed. I don't know if tablet support is fully fixed, as it's still not allowing me to configure the stylus - but that's another issue. Need to do some more testing to confirm the behaviour and i'll submit another bug if that's the case. Thanks for your help!
Thanks for the reply Jason, much appreciated. What's confusing for me is i'm not seeing any pointers in logs, or any kind of error messages. Where would be a good place for me to look to confirm that this is indeed what's going wrong? Thanks again! -B
Unable to Map Buttons on Cintiq 22HD