My opinion is that it's OK to make an AGI SCUMM game as long as you don't have to use a hacked interpreter to play the game (AGI Mouse, 256 colour AGI, etc). This because Sierra's AGI interpreter itself probably isn't legal to distribute anyway, and besides the old interpreter doesn't run well in WinNT/2K (which many people use, including myself - I hate Win9x, and haven't even tried WinMe), and not at all on any Unices.
In other words, it should be possible to play the game with Sierra's interpreter AND Sarien AND Nagi - if that is not possible with your AGI Scumm implementation, then you should drop it IMO.
Cheers,
Magnus.