Boudin or andoulle would be delicious in haggis.
Boudain Blanc, a French white sausage that the Cajuns eat, is almost identical to Haggis, save that Haggis has oats, and Boudain has rice. I wouldn't put it in Haggis (since it is essentially the same thing!), but I have served it as a substitute a couple of times when I couldn't get the real thing.

Cheers,

T.