emulate Photoshop Darken mode without minmax?

I’d like to simulate Photoshop’s Lighten and Darken blending modes. On recent hardware the blend_minmax extension makes this straightforward, but I need to do it on some hardware that doesn’t support the imaging subset (namely: Rage128, Radeon7000 through 7500.)

Is there any way to simulate min / max blending using multiple texture units? It seems like some combination using SRC_ALPHA_SATURATE should be able to do it, but I can’t figure it out. A cursory search of the forum here didn’t turn up anything either.

This is for building complex alpha masks prior to compositing, btw.