We would consider doing this but not as a direct upload to a Debian PPA, but rather a "check that your Ubuntu package will also build on Sid" capability. In other words, if folks upload to a PPA for the current Ubuntu development series, and the package builds successfully, we'd also build it against Sid.
PPA builds consume a great deal of resources and it's difficult justify spending that amount of money as a gift, especially if the net effect would be to encourage people to build packages which don't necessarily work with Ubuntu. But we can see the benefits of encouraging developers to factor both Ubuntu and Debian into their packaging efforts, so we're open to that.
While it may seem like little work, it's not something that is a priority and we have many, many other demands on the system. LP is open source, so this may be something that a person with the right motivation could contribute.