∗∗QUOTE
Which of those (probably out of #1 through #4) sound "good"?
One immediate "problem" I can foresee, with just adding a second side
to each pcb and then running traces (on the "new" side of each board)
to pin headers: Our pcb-making process is rudimentary and does NOT
include the ability to make plated-through holes. SO, to have headers
that are connected to the new top side traces, while the connectors
are also sitting on the top sides of the boards, I wouldn't be able to
solder the top side's trace directly to the headers' pins, since the
headers sit right on the boards. So I'd have to make an extra row of
holes next to each header row, that could have pins inserted that
could be soldered on both sides. (I was thinking I might just use a
single-row header, upside down with the longer pins halfway into the
holes. It might be "ugly". But it would also provide handy
test-points...).
∗∗ENDQUOTE
Thought I would mention...again...the possibility of using pin headers
on the ∗edge∗ of the board. No need to drill holes or worry about
soldering pins on both sides. Here's an illustration:
http://macetech.com/edgeheader.jpg