It would be nice if SourceGetVersion, Reason, RequestedBy and RequestedFor could be added to the list of values exposed by the "Get Team Foundation Build Parameters" action. There are others that might also be nice, so the ideal solution would be to allow the specification of arbitrary TFSBuild parameter names.
It appears that one possible workaround is to add the Variables attribute to the FBCmd element in the TFSBuild.proj file: Variables="SourceGetVersion=$(SourceGetVersion);Reason=$(Reason);RequestedBy=$(RequestedBy);RequestedFor=$(RequestedFor)"