Oh, right. I'm clearly confused on multiple levels here. (I erroneously referenced #829088 above instead of #835460, and I closed #835460 prematurely. And I mixed up 3.1.12-1 with 3.1.12-2. This might or might not be related to my daughter being born two days after I got the approval...)
I made some quick attempts at understanding how to use sbuild for oldstable, but I failed, so unless somebody would like to help me build the package in a clean oldstable environment, I think that we can close this bug. I don't feel that I have time or patience enough to figure out why sbuild doesn't work for me. :-(