It might be fine for NETBSD / L4 if the servers provided within L4 Env are GPLed but if the interfaces to them are not.
As for NetBSD/L4: don't expect GPLed code to make it in the tree. It wouldn't (and shouldn't) happen.
Is there any special reason for using L4Env as the foundation of a NetBSD port? L4Linux was written before we even started to think about L4Env so you should be able to port NetBSD based on the interfaces provided by the L4.
No, there's no special reason to use L4Env directly. It is not really critical, since the X.2 (V4) API, as implemented in L4Ka::Pistachio would be enough for a new (Pseudo-)architecture in NetBSD. The port would look somewhat like L4Linux too.
Anyway, L4Env would have been an interesting abstraction layer besides the clean MI/MD separation in NetBSD... Don't bother: not using L4Env in this particular context is not a show-stopper.
Many thanks,
-FH.