If present, the special node name "default" will match any node that
is not matched by another node definition (including one actually
named "default", if there is any).
I'm having trouble seeing how that will actually help you, though,
because you have to have Puppet already running on the client. Also,
it is tricky (but I think possible) to make Puppet reconfigure itself
in such a way that future runs would select a different node
definition. You would need to deal with installing the corresponding
cryptographic certificate.
John