If there's anything else I can provide that would help further diagnosing this
issue, I'll gladly try.

Steps I'm thinking about taking for further experiments, in the meantime:

1) try with latest stable kernel available for my distro 
2) try with liveCD of a commercial distro /w rolling release
3) try with older liveCD /w fglrx driver (perhaps from 2011)

I can change the radeon files to output more data or 'debug' trace too. I would
need minimal guidance for which DEFINE to set prior rebuilding to get that
debug trace.

Thank you very much in advance; I can't understand why this pro beast from 2010
can't drive that monitor through DisplayPort.