No, you will not brick your Wii if you try to run a PAL game on an NTSC Wii. No updates from the game image will be installed while using a USB loader. I'm not familiar with Wiiflow's default settings, but if you get black and white (or otherwise distorted) graphics, you might need to change the video settings to NTSC.
You'll brick your softmodded wii if you play from the wii disc channel and it requires update/patch...and you update. If there's no update required, you're safe to play via the disc channel.
Usually, to play PAL games on NTSC wii requires force wii/NTSC. With exception in my case, retail PAL house of the dead overkill doesn't need any forcing--just play as it is ie using PAL mode aka force disc or force PAL if you must. BUT this is if you use AV (red white yellow) cable.
If you used component (red green blue + red white) cable on a HDTV--which is most recommended, buy from ebay it's dirt cheap--you can play any PAL game without adjusting video mode. Just play.