Maybe. It depends on whether the resolutions are hard-coded into its driver ROM, or are in an extension that can be modified. For the built-in driver for the PM8500, I had to figure out which values affected the video parameters. Here you might just need to find what sets the dot clock so you can lower it. Once I figured out the built-in driver, finding the resolution tables in the TwinTurbo PCI card was a lot easier.

Can you point me to any extension/driver that's used for this card? I could take a quick look in case there's any kind of obvious resolution table inside. I wasn't able to find anything on the web after a few minutes of searching.