For some reason the Brink engine doesn't have the old command that has been in all id Tech engines since at least Quake 3. Com_maxfps, which allowed you to set a cap that the fps would not go over.

At the moment my fps jumps around between about 90 and about 250, which is quite annoying and makes it seem unsmooth most of the time, since as you know, the smoothness comes from the synchronization of fps and monitor hz. I don't want to use vertical synch, because it causes latency.

Is there a command to do this that I am unaware of, or do we need to star asking Splash Damage for one?

Try the Steam forums for Brink, theyre far more active than the tech forums here. Unless you start a request thread about wanting the feature(idk if it exists) then either here or the Splash Damage forums.