The discussion tells me, that the core of problem is not whether the licence is really violated or not, but that there are always people, which have bad intentions, people which look for small, irrelevant details and use them to destroy happiness of others (I don't mean You, narita, it is general statement). They should have as few opportunities as possible.
That's true.
So, why not split PlaneShift into 2 packages ? One with game engine and one with art ? There would be 2 installers, each would display different licence, nobody could say, that GPL software is incorporated in non-GPL package. It would be also more convenient for players and developers - there could be rare updates of art package and more often updates of game engine. Linux/Mac users which can compile binaries themselves could download new art at the same time, as Windows users, when new version is released. Does it make any sense?
You don't need that ...from what I remember when I studied the GPLv2 it's perfectly ok that a binary package includes GPL code together with non-GPL data and art - the only problem arises when the GPL code is packed together with non-GPL code. The only thing that that paragraph limits is that all
code that
derives or is
tied together with the GPL-code has to be the same license. Therefore there's no problem if art or data aren't GPL (frankly, GPL isn't really a good license for art and data). And also it isn't a problem if a non-GPL-compatible plugin exists for that GPL code - as long as it's not packed with the GPL'd code and doesn't include the GPL'd code in itself.
As for PlaneShift's art and data - they're under the PlaneShift licence of which the basic goal is that PS art and data aren't included in other projects (especially other similar games). Atomic Blue (="the PS devs") are the holders of most of the "copyright" rights on the data/artwork made for PS (which is also fine, since the author has to sign the Atomic Blue Contract before his art gets into the game), while granting the authors to still use their work unless they want to move it to another game, and the rest of us to use the data and art together with the (official and non-hacked) client.
In any case there's always the possibility that AB gives you explicity a different license or rights - if you're really lucky, that is

...hope that's clear enough.
BTW, what's wrong with "just" having an in-game cartography skill, that's planned to be included anyway? Why not help the devs make the skill that fits into the setting and game a lot better then a Google Maps mod. ...and probably easier to code as well...
p.s. if you're still so hot on making it and AB allows you to, why not rather tweak the NASA's World Wind code - that's open source.
edit: just made some layout adjustments