Okay... so after a little bit more fiddling with it, it looks like the culprit is actually the Highlight Size, not the specular color or specular value.
I set the Highlight Size down to 0.100, and left the Specular Value at 0.250, and the specular color to that same light grey, and this time it rendered up without any seams as well. I did also return the translucency to it's former state, with a skin-ish brown/tan color, and set the translucency value to 0.100 for all material zones being tested.
No seams with the highlight size set to 0.100... maybe Poser10 just doesn't actually want us to USE the highlight size at anything more than a .10? I mean, I know a skin should not have an overly large highlight size value, but 0.200 doesn't seem like it's that high for a highlight size on a skin. Nor does 0.250.
If I turn the highlight size even up as far as 0.150, the seam begins to come back. So maybe it's just this particular skin, or maybe it's just P10 and how it handles specularity on the skin.. I have no clue. But I guess that means I have to stick to a value between 0.100 and 0.150, preferably not actually reaching that .150, to avoid the seams..
I'm going to test it here with adding the specular and bump maps back in and see what it does... if it works, then it works I guess. LOL