It is not possible to copyright a data structure. It *is* possible to
patent such a thing, but the Z-machine has not been patented, so there
are no problems with writing games that use it. The ZIP interpreter is
freeware, so you can include a version of it in the distribution of your
game.
The relevant passage from the Inform manual is this:
A story file produced with the Inform compiler (and libraries) then
belongs to its author, and may be sold for profit if desired,
provided that its game banner contains the information that it was
compiled by Inform, and the Inform version number.
Which doesn't sound too onerous a restriction to me!
-- Gareth Rees