I really like the idea of them as wall art rather than something on fabric, their too special to just be sewn on something willy-nilly! I'm really jealous, that's a fantastic lot.
I think they might not be uniform enough for a portrait type thing but I think something a little more abstract would still be cool wall art while showing off the buttons more. For very different buttons like this I think the drought relief type thing is better than the portrait or anything where the buttons are split up. That way the piece is about the buttons rather than the buttons being about the piece. Good luck! and I hope you post the final product here so we can all see it!