Did this work?
I am having the exact same problem. All the American and Japanese games I want to run are in 50Hz on my USB-GD! Even Soul Calibur, despite the PAL version having a 60Hz option!
I made a topic on this in the Support sub-forum, since apparently the DC-X boot disc allows forced booting in 60Hz. Is that the one you used? Which one did you try that didn't work? I don't mind disc swapping if it works.
That region change mod looks terribly difficult in such a small space. I had enough trouble soldering a new controller fuse and time keeper battery in!
So basically, did you fix your problem? Because I can't play anything in 50Hz, it's horrid.
Also, since I bought my USB-GD direct, I had the guy's email and I've just contacted him. Hopefully he responds. Apparently the rival hardware, GDEMU, allows you to force games to run in 60Hz. So it is possible with an ODE solution.
And yeah, I'm running my DC over RGB SCART to a CRT. I have no interest whatsoever in VGA. Where does one even get old CRT PC monitors? I want to use lightguns!