Dev mode would not be needed for emunand anyway (even if devmode could load emunand), because firmlaunch doesn't matter in a cfw emunand. Okay sysupdater won't be able to update NATIVE_FIRM, but you never actually use the NATIVE_FIRM that's installed to nand anyway, firmlaunch always load a higher version regardless.
The reason why it bricks sysnand is because the 3DS doesn't use all those extra patches and hacks that CFW uses when loading. So if you updated while firmlaunch was enabled, then it wouldn't update NATIVE_FIRM, and then once you try powering on again, it can't actually run since NATIVE_FIRM is incorrect compared to the rest of the firmware.
If you were able to power on your 3DS and have it instantly load CFW + all patches including firmlaunch before sysnand even loads then it would work. This is basically what happens with emunand, it loads it with all the patches enabled including firmlaunch. But because that isn't possible that's why people get bricked when they try using sysupdater to update when firmlaunch is enabled.
You can update emunand by installing all 9.5.0-23 system cia's, but nintendo deleted all titles from their server which are older than fw 9.9, so you have to google to find an 9.5.0-23 EUR/J/EUR package, then install them with Devmenu, technically, you can use FBI, too, but FBI failed at some cia's installation when I tried, no problems with devmenu
Everything worked out great. Installed sysupdater 4.1 cia with FBI. Put the 9.5 update files (many .cia's) into update folder in root. Ran sysupdater and it updated, rebooted itself and it came back on 9.5. All must be done in emunand (obviously).