Thanks to everyone for their responses. Let me try to clarify my original question (it's not that I don't like the original answers, but I'm still searching for something...)
I know that the C chord is about the only chord you can fully play on a diatonic C box (thanks to Larry for the list of chords & notes), and that when you "play" a G, D, F, or whatever, chord you're only getting some of the notes that make up the "full" chord. I'm assuming that if you're playing a D note in a tune (say pulling the 3 and 7 buttons for an octave) that you could also pull the 5 at the same time and give yourself a D chord. That makes sense to me, and when I mess with it on tunes it generally sounds okay. The question I think I was really driving at is a little more complicated - at least to me. If you're playing an A note(pulling 5 and 9 for an octave) does it make sense to add in the 7 and get a D chord. The note you'd be playing in octaves is an A, but it's possible to turn that into a D chord by pulling the 7. Or is it dopey to insert a D chord when the note you're looking for is an A? Same thing with deciding between pulling a single note on the 6 or 7 button (an A or an F?) or pulling both and getting a G chord. Is it all a matter of "that sounds cool" or "that sounds lousy"?
I am listening. Really I am. Lots. Many thanks, as always.
i think if you are aiming to play something
like an A chord, you had better not hit the D note
if you do [music theory] it instantly either
sounds like A-sus4, which is a very weird chord,
or it sounds like you are playing [part of the]
D chord, which does NOT sound right if everyone
else is playing A
adding the G note to a partial F chord
[AFG] is going to sound weird too
it suggests an F9, which has its place in
some music, but not really in anything like
cajun or zydeco
what you need is a book called 'chord chemistry'
which tells how to make different chords and
what they are usually used for
you can try anything you want, though, it might work
what will stick out though, if you just
do one weird chord, is the fact that it;s the only one
usually guitar and keyboard players throw in
9ths, 2nds, suspendeds, but they have the ability
to do it all the time, you will only have
limited opportunities on the one row
and they are not usually playing simple folk music
when they are doing that