[Groff] how to write this section-break macro?

[Dorai Sitaram]

I'd like to write a section-break macro, say .SB, with the following
properties:

(1) If the break occurs in the middle of a page -- i.e., with pre-break
and post-break text on the same page -- then the break is representated
as some tell-tale vertical space.

(2) If however the break occurs at a page break, then the new page
starts with an ornament, such as a centered ***.

The rationale for (2) is that we need some indication that the section
break occurred -- since there is no vertical space marking it. 

Any ideas would be much appreciated.  I think groff must have enough
registers that can be checked to pull this off, except I don't know
which.  

--d