If I'm understanding your question correctly...

Unless the vendor morphed both the eyebrow body part and the head there won't be a match, you can check by toggling visibility on the eyebrow body part. I'm going to assume that you've already discovered that the visible brow body part in fact doesn't match. From my experience with using commercial character morphs the brow body part isn't exported for the custom morph process and consequently doesn't match, there's really no way to remedy this. There are exceptions though, I think that Fabiana often creates her morphs to include the brows for instance.

If the head morph is derived solely from dial spinning the Daz morphs there should be a match so you wouldn't be experiencing this issue.

BTW use external binary morphs has zero to do with this issue and has nothing to do with "a chronic problem in Poser"... Most vendors simply don't create their custom morphs to include the eyebrow and don't bother with morphing said body part along with the head body part.

You could however create your own morph from dial spinning or in an external modeler (including the brow body part) and your custom texture will blend fine with the morph... Warning though, morphing the brow to match the head is a royal PITA and you'll be battling some kind of offset distortions more than likely. Probably why most don't bother, and the fact that few use the brow body part.

Being that you are using the brow area as transmapped you could try just pushing and manipulating the brow body part to some kind of semblance of a match with the putty tool (or even an external modeler, export the head morph as guide)... there's less geometry that needs to match and the brows would probably look ok with a bit of offset, just an idea.