Okay, I hesitate to share this info with you because of the potential hazards, but like a terrain 5 rated cache you can decide what is in your best interests.
It is possible to create your own custom Garmin maps and upload them with MapSource to your GPS. The only risk is you can render your GPSr into a nice dash ornament if you do something wrong. If you are not happy with that kind of risk, read no further.
Step by step instructions are available here: http://www.gpsinformation.org/adamnewham/article2/
The example shown is building a golf course map for your GPS.
Another site here: http://www.keenpeople.com/index.php?
I've used the MapEdit (http://www.geopainting.com/en/) approach and found it to be intuitive and very slick. You can import shapefiles (one of the most common GIS formats) and do customization and labeling. You can also load raster (image) layers like orthophotos and then digitize features over top of them.
Anyway, hope this potentially new found power to create maps doesn't distract too much from caching.