Ended up making it work (loading a backup!)
The super-clean (cleanest to my knownledge) procedure to get a 4.2 new wii to work with backups/undubs/etc:
1. install bootmii/hbc
2. use bootmii to backup NAND (can never be too safe). Remove the SD and copy that backup to computer/safe place.
3. use cboot2 (first installer, then loading it) to boot latest trucha bug restorer
4. tell trucha bug NOT TO RELOAD IOS (as it'd destroy the nice work cboot2 did). Truchabug overrides startup libogc loading of IOS, so we're fine. ios36 menu. Change target to 236 (this way we don't overwrite anything at all). NAND permissions patch YES WE WANT! (as the IOS we created will be used solely for hacking
version 6b from homebrew menu. Load IOS 236. Delete from system stuff the IOS249 (nintendo put a stub there in 4.2 to sabotage homebrew. It's just a stub.). Ignore stupid errors/warnings from the program and proceed (it scared me here too, but worked).
6. use the latest cios installer, loading IOS 236 on startup too.
7. boot a game with your favourite loader
8. make another NAND backup just in case