Hi,
If you name your script 00foobar.source it will be executed first _and_ sourced, therefore all variable definitions from 00foobar.source will also be visible in all other scripts. You might need to export the variable to have it available in the entire FAI run, but that I'm not sure of.
AFAIR this doesn't work - fai-class runs in a subshell so any variables you export in class/*.source will only be available during fai-class execution. Workaround is to write a temporary file which you later source from class/SOMETHING.var.
Regards, Michael