http://line6.com/support/page/kb/_/effects-controllers/fbv-mki-mkii-controllers/the-complete-line-6-pedal-compatibility-faq-r58
The FBV3 and FBV Shortboard MkII seem very similar functionally and have a great enough overlap in supported amps to suggest it's not going to completely fail to communicate, and would be very unlikely to damage the amp.
Are we simply dealing with a support demarcation where it mostly works but compatibility might be broken by an update at some point? Or can it not communicate already? Yes we know it's not "supported", but what will happen? Has anyone tried it?
(I have both a Firehawk 1500 and a Flextone IIIXL, and am trying to decide between the FBV Shortboard MkII and FBV3... and I want the coloured indicators for the Firehawk.)