I suspect it MAY be possible by tricking an update that has a topo IMG file instead of the normal one.
I found this method for updating the City Navigator map, so just maybe...
http://www.ramforumz.com/showthread.php?t=160470
Their method looks like it might work, and I suppose the only way to know for sure is for someone to try. If it fails though, it sounds like it has the potential to screw up the programming of your head unit really bad, and considering the GPS interface is pretty limited (designed for road navigation by address) I don't really think it would be worth the risk and effort.
PS. Am I allowed to post that link?
You need to cut and paste the text over like this, so we don't have to jump back to a different forum to read it. Personally I'd post the link where you got it from too, if for nothing else than to show you're not making it up.
1. First you need a copy of the radio software update. (I used the last 50.00.07) It will be usually in .ISO form. I found it easily doing a google search.
2. Download the latest copy of Garmin City Navigator North America NT Unlocked. (I used 2014.10) You can find it on most torrent site.
Here is the tricky step:
3. Mount the .ISO file on a virtual drive (I used Daemon tool. Its free) and open it to see its content. Copy all the file to a new folder on your computer. Lets call this Folder "Radio Update"
4. Copy the file gmapsupp.img from the newly download folder "Garmin City Navigator North America NT" to the folder called "Garmin" inside the "Radio Update" folder. Once copied inside the "Garmin" folder, rename it gmapprom.img. (the one inside the "Garmin" folder)
5. (optional) if you have the junction view files, create a folder called JCV folder inside the "Garmin" folder and copie the JCV file in there.
6. Now you need to burn the content of the "Radio Update" folder on a DVD. Burn all content and not the "Radio Update" folder.
7. Insert the DVD in the radio unit and wait for a long time. It took me almost 2 hours to complete all updates.
Do this update with the vehicle running or a charger on the battery. You can not interrupt the process. Starting the vehicle will interrupt the process.
It did not ask me for a code for the maps.
This process updated all my maps and the radio software at the same time.
What aftermarket unit would support topo maps or custom maps? It would be awesome to overlay the local offroad park trail maps onto google satellite maps. I know Garmin software supports this on their outdoor handhelds.
There's one aftermarket head unit I've seen called the JK1001 by Insane Audio. It's made specifically for Jeeps (there are JK and TJ versions) as a direct swap in for the Chrysler head unit, and will even integrate with the Jeep's connectivity package, steering wheel control buttons, etc. Comes with a bluetooth OBDII reader as well, so it will let you see all your engine data real time. And it's Android based, so with a wifi connection you can download any Android app to the head unit you want, and has a few map programs as well as Torque Pro (for the engine data) preloaded when you order it. Personally, I don't think it's worth the $1400. It doesn't have Sirius, which is a huge deal breaker for me since I live in an isolated area (although apparently they are now working with them to get a bluetooth module and / or app developed for satelite radio). And for anything my 430n can't do, I can download the same apps to my cell and just use that. I also already have the same OBDII reader and Torque Pro on my phone too, though I'm thinking about getting a Trail Dash anyway. Does way more than Torque Pro will and lets me use my phone just for the GPS.