you can not only chose from the list but also insert any cmdline you need to run, so you can hook in any diff/merge tool that supports being called with parameters. e.g. I use this for diffing:
NxNMerge.exe %1% %2% /title1:"%3%" /title2:"%4%"
For a description of the parameters open the Tools->Preferences->Diff/Merge in AccuRev and click on the help button.