I've noticed that BeyondCompare3 doesn't turn up in the diff/merge tool dropdown selector in the Hg Workbench on Win7 x64. The reason for that is that MergeTools.rc only knows how to find it on 32-bit Windows installations.
The following line needs to be added in the BeyondCompare3 section:
beyondcompare3.regkeyalt=Software\Wow6432Node\Scooter Software\Beyond Compare 3
(and for BC2, if anybody is still using that)
beyondcompare2.regkeyalt=Software\Wow6432Node\Scooter Software\Beyond Compare
With these additions, the tools are detected correctly.
Thanks for your consideration, Tim