Well, IANAL, but:

they might be able to issue a cease and desist order, cause you're mucking with their property.

OTOH, until you actually do something like selling or distributing a program based on Bryce 5, they might also not have any basis to do that.

But as I said, IANAL, especially not a corporate one, so if you have any acquaintance who knows something about the matter, consult them.

BTW, take a look at this earlier thread about the Bryce 6 wish list. There's also a link to a wish list of mine inside.

-- erlik