gerwin:
I played around with this once with my LCD (and nVidia drivers), and my attempts to change the fullscreen refresh rate through the display settings or lockers had no effect. Testing kept showing that I was actually at 75Hz in fullscreen no matter what I did. I think you can get some better success with your CRT.
The emulation refresh timing within dosbox is independent from the host refresh rate. I'm a little confused by what exactly you're trying to do, so it could be good to get some background on what's been discussed before. If you're interested, there was some discussion about it in this thread (which is about a year and a half old). My posts in the thread go into some detail about it:
Screen tearing in DOS games running through DOSBox
and I once started working on a patch related to it in this thread:
Vertical retrace sync patch
I'm considering revisiting the issue sometime (h-a-l-9000's recent experimental vga patch got me interested), and releasing a directx-only patch. I lost focus a bit last time..