Yes, I installed 1.9s earlier this year. No problems. (except for the Mario Kart “semi-bricking” my Wii, which I told you about on Geoexpat 😉 )
I also have a US (NTSC) Wii modded with Wiikey. I got mine at Ho King Commercial Center in Mongkok, Dundas Street. (near Starbucks/California Pizza Kitchen)
The key to doing Wii Updates AFAIK is to ONLY do updates from games whose region is NTSC-U, since your Wii is from the USA. In my case, I used a Japanese Mario Kart to do the upgrade.. which is a big no-no. I couldn’t get into my Wii Settings page after doing that.