Isolated 1-10V (or 0-10V) output
I'm not sure I'm following what all of your constraints are, but my first thought would be something like this:







simulate this circuit &ndash; Schematic created using CircuitLab

This is similar to the previous idea, but it now uses an LMV431 shunt regulator to deal with the variability of the current source. If the switching frequency is zero, the control voltage will be 1.25V, and as you increase the switching frequency, the control voltage will rise.