(Darren) That's correct. The whole instrument would be flat and it wouldn't sound wrong but any other instrument accompanying it would have to tune down a bit. Rather difficult with a piano. One way round it would be to have a small bleed of a lighter gas into the air the organ uses. You could use hydrogen, helium, neon, methane, ammonia or hydrogen fluoride. Perhaps I should patent this lunacy.

Wouldn't some of those risk producing the Amazing Exploding Organ? That puts me in mind of the Large Hot Pipe Organ which produces sound by exploding a propane/air mix in its pipes.

(Darren) It's blown itself to pieces and taken the website with it. I drew a blank with your link, alas. Helium would be safe enough and it's much cheaper than it used to be. It would be amusing to use sulphur hexafluoride, totally inert and non-toxic. It would lower the pitch by over an octave. I think I'd better shut up now.

Large Hot Pipe Organ (thanks to google), with MP3s (although the site doesn't seem to have the bandwidth to supply them very well).

My curiosity is partially because I'm playing carols at the village Christmas tree tomorrow evening with the local brass band. Getting 40 or so of us in tune may be interesting if we get a cold evening.