I did manage to get things sorted out last night, but I am still curious...
I had both GPO4 and Finale 2010 installed on my system, with their associated ARIA libraries. Everything was fine, and I could get to either library through the stand-alone ARIA, the VST ARIA plug-in in Sonar 8, or Finale.
After I installed JABB3 the other two libraries disappeared. The ARIA player was, it appears, upgraded as part of the installation. The actual libraries were still intact. So I re-ran the GPO4 installer, and let it install ARIA and the GPO library, but not the samples. Everything returned to normal.
For future reference, did I do something dumb?