There was another bug: I had to change the single quotes around the sed command to double quotes so that $url could get expanded -- that required using "\\\\n" instead of "\n" to make a newline.

If you want to follow along, bear in mind that things that look like $(XXXX) are expanded by make. Shell variables look like $XXXX, but to get a dollar sign into the shell portion of the recipe you have to double it -- $ is a make variable -- constant, actually -- that contains '$'.

The final make recipe is:

post:	pre-post
	url=$$($(POSTCMD) $(ENTRY));					\
	   sed -i -e "1,/^$$/ s@^$$@Posted:  $(POSTED) $$url\\\\n@" $(ENTRY)
	rm -f .draft
	git add $(ENTRY)
	git commit -m "posted $(ENTRY)" -a