Source with variable output impedance / Fast large signal voltage variable resistor
The best I've come up with so far is to use a series of switched
  resistors. The downside is the large stray capacitance, which severely
  limits the BW in the high resistance mode.


This is the problem in a nutshell - if you can't achieve it with conventional resistors then semiconductors (or anything else that might spring to mind) just won't work either. 

You need to re appraise your requirements or look at things in a different way.