Update:
Went to the session and asked my friend to sing the song, explained about the request. He did. (Major key, somewhat complicated tune for folk music, and a lot of gratuitous harmony since everyone here knows it.) Here's David's shorter version:

"on Mon we had bread and gravy,
On Tues was gravy and bread,
On Wed and Thu was gravy and toast,
Which is nothing but gravy and bread,
So on Fri I went to the landlord,
To get something else instead,
So on Sat morning by way of a change,
We had gravy without any bread.

David says it's a song his family has always sung - he learned it from his mom when he was little, and he's close to 70, if not already there, so the song goes back at least to the '30s. His mom still lives in the family house in Canada - Quebec, I think - and he said he'd ask her about the song next time he talks to her.