Passing around build parameters between builds

I want BuildA to trigger off BuildB, BuildC, ..., BuildN and I want to pass a string around between all the builds.

I don't think that I can use artifacts because I need the string in the commandline arguments of BuildN so I think I have to use Build Parameters.   I created a Build Parameter in BuildA (called system.foo) which is run with a specific value.  How do I make BuildN etc run with the same system.foo=foovalue?

I've tried createing the same build parameter on buildN with a default value and having BuildA check in teamcity.default.properties in an attached VCS root with the contents being system.foo=foovalue, but if I add an echo %system.foo% to the commandline steps it always prints off the default value and not the value that was checked in teamcity.default.properties.

Am I missing an obvious step somewhere?  Any help would be greatly appreciated...

Cheers
Dave