You just have to know and trust the breeders for being reputable. Even if they may or may not know, it's a risk you have to take with boas.
I have 3 boas, though only one is a BCI and has been in the same room as my pythons for over a year. When I got him, I only had a few pythons, and nothing has come up and killed them.
Here's the thing that quirks me. The boas are asymptomatic for years, correct? But can they not still transfer the disease immediately to pythons that will show effects within 6 months?
Therefore, if breeders have pythons and boas, then they can basically determine that their boas don't have IBD, because they're pythons are thriving.