Okay, here's how you can do any of this through mset:
1. Pick whether you want to boot a (Gateway-encrypted) Launcher.dat or rxTools.dat and grab the appropriate code.bin - or both.
1a. code.bin for Launcher.dat's:
http://s000.tinyupload.com/index.php?file_id=47753777289998764895
1b. code.bin for rxTools:
http://filetrip.net/dl?YMiN8kg7dh
2. Put whichever code.bin on the root of your SD card, or grab both and name one something like rxcode.bin and the other gwcode.bin
3. Put the SD card back in the 3DS
4. Clear your 3DS internet browser history and cookies
5. Point your 3DS browser at
http://dukesrg.no-ip.org/3ds/rop/?LoadCode4.dat&code.bin (you can change the "&code.bin" part to the name of each code.bin, i.e. &rxcode.bin or &gwcode.bin, I made bookmarks for each)
6. Wait for the browser to crash (should give a "Please save what you were doing and restart" message)
You now have mset installed for whichever thing you want - the rxTools code.bin will launch rxTools.dat, and the Launcher.dat code.bin will launch any Gateway-encrypted launcher.dat (that is, if something gives you a Launcher.dat and a Launcher_GW.dat you're gonna need the Launcher_GW.dat renamed to Launcher.dat)
NOTE: If you followed those tweaks a few pages back to insert the slot0x25KeyX.bin into rxTools.dat, you can't use that modified copy with this, it won't boot. I'm SO glad that was brought up, because otherwise I'd still be puzzling that out, hahah