Hi Jan,
I guess what you are describing is commonly termed a VCO or LO Buffer. If so the simplest design is differential pair (cascoded/cascaded or other variant) based with resitive load, such that the "limited" output swing is I*Rload related. Given your high frequency requirement and large capacitive load, the current needed to "limit" at 10GHz would be large.
If you wish truly to control the VCO amplitude, then you can use ALC (automatic level control) techniques which compare the VCO amplitude to a known (bandgap) voltage in a feedback  system which controls the VCO current in order to keep the vco amplitude fixed. This would be suitable for driving your mixer VCO port directly if you had no requirement for isolation/ mixer on~off loading.
Anyway, I would check with the mixer design whether the mixer gain would actually benefit from limiting or whether the key parameter is slew (which is usually the case is switching LO type mixers), I think getting the same gain over your frequency range from you mixer would be tough even if you did have limited VCO signal...
Cheers
aw