If you want to try to support the last two or three haskell platform versions, you're committing to supporting GHC 7.4.x and bytestring 0.9.2.1 still. Debian stable still has 7.4.1. Plus there are one or two things that are in blaze-builder that people use that didn't make it into bytestring. So I agree with Kazu, I'm not ready to switch to bytestring builder yet --- especially since 0.10.0.2 was kind of botched.
I don't like preprocessor tricks here. If the public haddock docs on Hackage are built on a new machine but the user's machine is old, confusion is sure to result. You're forcing your downstream users to put in all of the same #ifdefs you used, too.
Sad though it may be I think we just have to wait a year until everyone's on a capable version of bytestring, then deprecate blaze-builder and make the switch as quickly as possible.
G