Can you make (or buy) a broadband transformer with a center-tapped
secondary, and use a pair of Schottky diodes arranged just like a
full-wave rectifier? At your low power level, I suppose you'll want at
least a 1:1:1 transformer ratio, and 1:2:2 likely will be better. Some
experiments or careful Spice simulations with good models should help
optimize the conversion efficiency, which won't be wonderful at those
low levels because of the drop in the diodes. Be sure to provide a DC
return path for the diodes or it's not going to work.

Since that balanced push-push circuit suppresses the fundamental and
third harmonics, you can get _nearly_ an octave of coverage with a
single set of good filters, but note that 2*500MHz = 1000MHz, and
4*500MHz will also be generated, yielding 2000MHz output at the same
time, which would be passed by your filter. Will that be a problem to
you?

Cheers,
Tom