Okay, first off, I'm not going to explain how to get yourself an ISO of the latest maps. If you don't know how, sorry.
Now that we've gotten that out of the way for the 1%, I've tested and found that yes, you CAN use Toyota/Lexus Gen4 GPS data discs in the OEM Subaru Nav system.
I believe the most recent is the v13.1, which were released in October 2013. BitTorrent is your friend in getting these. A search on an appropriate website of "Toyota-Lexus Gen4 v13.1 Navigation DVD - NA - U28" should get you to the correct one. You want to make sure that it's for a Gen4 system and that the description says it's for a bunch of Toyota and Lexus vehicles in the 2004-2007 model years.
Once you have the ISO file, you'll need to burn it to a disc. You'll need a DVD+R DL disc. I wasn't able to get a disc that the nav system liked using Astroburn Lite, however, with ImgBurn, I was able to use the option in the bottom right of the main window to 'Change Book Type', then in the LG tab, select DVD-ROM and burn away.
Simply boot up the nav in the car and once it's running, swap out for the newly burned disc. On a side note, it appears that more data fits in a smaller space now, as there's only 1 disc needed for all of North America instead of the two discs needed originally.
No more being unable to set a course for a new location!