I had a random thought a little while ago that any unreleased material (such as the mysterious box set) would come out under the Hell Interface alias.
Stylistically, a lot of the older stuff (at least what we've heard) is all over the place, and we know how much they take pride in ensuring the Boards of Canada albums and EPs are musically cohesive and thematically concentrated. Putting it out under HI would give them more freedom to compile this material any way they choose without impacting their legacy.
And for the record, I don't mean to be negative in the slightest. In fact, Old Tunes Vol 1 is probably my favourite thing they've ever done. I'm merely approaching this situation from a branding/marketing angle