I'm running it on two different NTSC-U wiis with 1.9s Wiikeys with no problems whatsoever.
What I did both times:
Run Dual Channel Remover v2 (I had japanese channels from NTSC-J Brawl still)
Update to 3.2U via WiFi (I still have IOS36 after this update, as verified by running Dual Channel Remover again)
[At this point, I updated to 1.9s, but there's probably no need, just listing here for completion. Ran the 1.4 config disk to make sure region free and everything was still enabled]
Popped in Mario Kart Wii PAL, unpatched in any way. Installed the update as prompted.
Dual Channels installed, ran Dual Channel Remover V2 again to see what would happen. Dual channels are detected and deleted, but you will be prompted to update again when you run MK.
Installed the channels a second time, ran MK Wii @ 480p widescreen. No problems, DREs, or anything. Runs at 60FPS in single and 2 player split screen. 30FPS online with 2 player split.
Online worked for a few hours before Nintendo brought the servers down.
NOTE: Ran it in 480i over component and over composite. Both cause the game to run at 30FPS instead of 60FPS in Single Player mode on an NTSC-U console. Strangely, 2 player split-battle (didn't try versus race) works in 60FPS. Game plays in 30FPS in battle (iirc) and races over Wi-Fi when you bring a second player online with you. Didn't get to test single player Wi-Fi.
Also, i just remembered I fumbled through the Wii settings menu too, worked fine and no "semi-brick". Just dual channels, same situation as running Brawl JAP on NTSC-U console. Also have ran NTSC-U brawl and gamecube discs since, all work same as before.
In conclusion, now that dual channels are reversible, there's no real harm in installing the update given you're already at 3.2U