Actually, with a little work, you can do some neat things – within the constraints. The closest I have come myself, as a non-CSS expert, is to take advantage of the capability to use HTML in the field label.
So I have added some dummy columns to my list, and then included those on the form where I want the divider to be. Then the dummy column's field label includes something akin to <HR>. It can look pretty nice – HOWEVER – the divider only spans the field label side of the form; the right-hand column where the data shows doesn't have the divider.
I signed back into this forum though to post some slightly related tricks that we have employed. See my post which I hope will appear in a few minutes.