That did it, thanks. I found the info at
http://msdn2.microsoft.com/en-us/library/aa833231(VS.80).aspx
Some extra attributes are required on the extensions line:
<extensions assembly="CustomCondition, Version=1.0.0.0, Culture=neutral, PublicKeyToken=<enter key here>" version="1" xmlns="urn:Microsoft.VisualStudio.TeamSystem.Data.Extensions"...