Quote - You mention splitting the edges as an alternative to using a bevel. I'm not sure of the terminology, but are you talking about material groups here?

Not material groups, no.

Imagine two flat planes, one horizontal and one vertical, which share an edge to make a right angle. Further imagine that the edge in question is made of a string of vertices, which are shared between the vertical and horizontal planes. Poser will assume that both planes are part of the same surface and try to smooth them according to its own internal algorithms (now somewhat controllable by the smoothing angle setting, but still...)

If each vertex along the edge is duplicated, so that there are separate vertices for the vertical and horizontal planes, Poser now "knows" that the surfaces must be kept separate and doesn't attempt to smooth the edge.

This process is often known as "splitting vertices". UVMapper Classic has a function for this, but it actually splits all the vertices in the model which is not what you want if it has any curved surfaces at all. UVMapper Pro enables you to set the angle which will trigger the splitting, along with other useful features.

Quote - I vaguely recalled some warning about faces with 5+ sides.

That was probably my vague warning. :-) Poser 3 used to actually crash if it as much as thought it was going to get a 5-sided polygon. Later versions are more robust, but the suspicion lingers that they are best avoided.

Quote - The concave N-gon issue - that applies to concave quads as well, doesn't it?

Well spotted, grasshopper. :-)

Quote - Send a file to the contact address on your website. Which file? I think Blender can export most formats, or would just the .obj be enough?

Yep, just send the OBJ file to my contact address and I'll take a quick look.