nVidia drivers (at least newer-ish ones) have "Create Custom Resolution" mode that includes refresh rate control among other things. I don't know if this is keyed to specific hardware, driver generation, etc. I double-checked it existed on my QFX1700, but the nV website's demo uses a GeForce FX with "old style" control panel (mentions ForceWare 60 in the article, which has pretty wide-ranging support):
http://www.nvidia.com/object/custom_resolutions.html

RefreshForce can do this but it can't create any custom modes. If that doesn't work, try RefreshLock. The official website seems down but these utilities should be easy to find anywhere.

On Voodoo Banshee and up, V.Control has some custom mode/refresh rate lock functionality too (never tried it though).

I found an even better way to do this with the nVidia drivers but I think it works on Win7 only.

obobskivich wrote:

nVidia drivers (at least newer-ish ones) have "Create Custom Resolution" mode that includes refresh rate control among other things. I don't know if this is keyed to specific hardware, driver generation, etc. I double-checked it existed on my QFX1700, but the nV website's demo uses a GeForce FX with "old style" control panel (mentions ForceWare 60 in the article, which has pretty wide-ranging support):
http://www.nvidia.com/object/custom_resolutions.html

This feature doesn't work on all official Forceware drivers for Win9x. 😵

Custom resolutions does not work reliably even with the latest nvidia drivers. The main problem is that they don't allow settings that are beyond the DDC information the monitor reports. It works in test mode, but is not added as new mode.

The most reliable way for win98 is to take one of the default monitor inf file and change the timing information therein to just allow the mode you want.
Then install the monitor driver and the graphics driver can only display those modes.