Why not an inline isolator/circulator? I've seen them used in some high power RF amps for preventing those reflections for damaging the less high power rated parts. Of course you're going to want a DC block as well if there's going to be risk of high DC voltages applied, but if it's applied suddenly, that full DC offset is a single AC pulse before the voltage stabilizes, so even with a DC block, if the switch on is sudden, it can do damage past the capacitor.
The lower tech, lower efficiency version could also be a high power inline attenuator. For example, a 10W 10dB attenuator would reduce your generator's output, but would also reduce any reflections by 10dB, giving your generator a lot more reverse protection.
Clamping diodes could work great too, but I don't know how low capacitance you can find for high power packages, so they may not be well suited to your application if your frequencies are high enough.