Converting to a single-color image (duotone sans black channel)

Hello,

Apologies if I'm missing something obvious, I'm trying to work out how to convert an image to a single colour - much like grayscale but colorised. Effectively what I'm trying to do is replicate the effect of Photoshop's Screen blending mode when the top layer is a solid color. I've put up an example to compensate for my inability to articulate this properly here: http://sho.ch/otterconvert.jpg

I can do a duotone easily enough but this has me stumped. It would also make my life notably easier if it was a single invocation of convert. 

Any guidance would be greatly appreciated. 

Kind Regards,
Alex