grep piping into sed, replacing inline; but I want sed to print the filename and changed line. Is it possible?

One possibility would be to pass the output of grep to a separate sed filter.

grep -l FOO "/Users/gjtorikian/blah/"* |
{ tee /dev/fd/3 |
  xargs sed -i -e '/FOO/{' -e 's/FOO/BAR/g' -e 'w /dev/stdout'
} 3>&1 | sed 's/:FOO$/:BAR/'

You could make sed print the line number (with the = command) when it finds a match and do further postprocessing.

It would probably be clearer to use awk.

for x in "/Users/gjtorikian/blah/"*; do
  awk '
    sub(/FOO/, "BAR") {found=1; print FILENAME ":" NR ":" "BAR" >"/dev/stderr"}
    1 {print}
    END {exit(!found)}
' "$x" >"$x.tmp" && mv "$x.tmp" "$x"
done

As I'm not native English speaker I probably didn't get it.

To 'grep' a directory you need '-r'. Usage of '-l' prints just filename and it stops grepping after first occurence.

# pattern=/home ; grep -l "$pattern" /etc/[a-z]* 2>/dev/null | while read line ; do echo "$line:$pattern" ; done
/etc/adduser.conf:/home
/etc/fstab:/home
/etc/fstab~:/home
/etc/libuser.conf:/home
/etc/mpd.conf:/home
/etc/mpd.conf~:/home
/etc/mtab:/home
/etc/netscsid.conf:/home
/etc/passwd:/home
/etc/passwd-:/home
/etc/sudoers:/home
/etc/sudoers.tmp~:/home