I haven't used a mobile device recently to access ML- but I'm using a user-agent spoofer on my computer and it still works fine.
Oops, actually I just used my iPad a few days ago and it was fine logging in through safari.
Not sure what's up.
It should be fine if you're only using the browser to log in.
The thing is, the GREE application has your user name and password stored so that when you click on the link to go into Million Live it passes it as authorization so that it can let you into the game.
Since GREE is no longer supporting customers outside of Japan, you won't be able to use the app (if you're using the non-JP version) to authorize your session. You can still use the browser outside of Japan though.
Have you tried using VPN or proxy to bypass the region lock? maybe that will solve this problem.
I'm not sure if this would work, but good luck! I hope you can get back in.
edit: As a quick heads-up, GREE logged me out this morning, but I still have my browser version of the game up.
However, my GREE UI is in Japanese, so I'm going to try logging on later today and see if it still works for one of my other games.
edit2: Yeah, I just logged in and my account appears to be just fine because everything is in JP. Guessing VPN would fix it!