Clamping colours AFTER blend

Is there any way of clamping the fragment colour to a value such as 0.5 after its been through the blending stage?

Clamping the alpha would not be any use, as the alpha is only used in the blending stage. I don’t want the colour to be rejected if it’s over 0.5, I want it to stay at 0.5 and no higher.

Rendering an additional polygon over the rendered geometry using another blend func would be no use to me, as I’m rendering a lot of geoms, each of which needs to leave fragments <= 0.5 in the frame buffer.

Thought not.

This sounds like a job for NV_register_combiners!