Quote - I honestly have no idea how it should work. All I know it that at present our IBL's in Poser only influence the diffuse aspect of any material when using HDRI's, and it'd be nice to have access the specular as well. Maybe this just won't be possible in Poser (or maybe I don't understand HDRIs.)

It's not possible in any software. The feature you're talking about actually works with reflection, not specular. You start with a normal HDRI, do a diffuse/specular convolution on it in HDRShop (or similar apps) and use the resulting images on a sky dome. When this sky dome reflects in your surface, it appears to have a specular. The size of the specular can be controlled by choosing appropriate values while doing the convolution. But what we usually call 'specular' as it is found in the phong or blinn nodes plays no part in the whole thing. See:

Diffuse and Specular Convolution