Offline map app capable of using Garmin Custom Maps / KMZ?
You might want to take a look at Locus Maps: It not only can import KMZ files (I guess the ones you mean keep to the Keyhole standard, so they should work). You can even have media (pictures) embedded within, and use them in offline mode. Maps also can be downloaded right from within the app, so you can be prepared for your off-road trip.

It also supports a bunch of different map formats (including vector maps), so I would wonder if you were unable to get your mentioned material loaded with Locus.

Besides: excellent support, active forum, and much more... Definitely worth a look.

And no, I'm not the developer placing my ad here :)