I would like to include a few source files
as 'executable' sections in a .cabal package
description. However, although I do want to
use main=mainDefault features, I do not want
those packages to be installed when I run
'Setup.hs install'.

Yes, just use this in the executable section:

buildable: False

But I do want it to be built (so Setup will
check dependencies, allow me to './Setup clean'
etc.). However, I would like './Setup install'
to install just the main library.