"....but the problem is the brush can't go over seams of the mesh, for instance, the seam that attaches the toes to the foot."

Neither can ZBrush or any other modeller.
Not on a "life" Poser figure, because Poser will break up the figure into sub-objects when it loads it from the figure library.

If you need to work across groups, just import the original object file from the geometry folder.
Once you're finished morphing with the MorphBrush, export the object file and then re-import it.
(So the morph becomes permanent)

Then use the SetUp room to turn your morphed object into a figure.
(Takes 20sec and three mouseclicks)

Once you have turned your morphed mesh back into a working Poser figure in the SetUp room, use the MorphBrush again to do the detail work on the head, adjust expressions to the new headshape, and make fix-morphs to get accurate joints.

After that you can use your morphed figure "as is", or turn everything into a morph for the original figure it's based on so you can legally redistribute it.

I made several full body morphs with the morphbrush this way, and I can't think of a single time I rather would have used ZBrush instead, because it's so much more convenient to work directly in Poser with a fully articulated "life" mesh.