Back in the time before Git, I would have strongly advocated that we do maximum effort to never shutdown the "master" branch, when it was more or less hosting all the development.
Now, we have pretty nice branching, and PRs, with builds on PRs... so that developing on a branch provides everything necessary.
But I imagine that what I mention as "everything" is missing at least 1 thing, or you wouldn't have asked ;)
So the question is: what do you think is missing during this time which makes you less efficient? "Just" a complete SDK build including some extra commits you'd like to test?
If it is so, I'm afraid it would be hard for the project to provide it as Alex mentioned: maintaining 2 build branches in parallel is beyond the current human capacity of the project, and contributing to SimRel/EPP and then starting a new versions does actually require a lot of tedious and error-prone work, which can itself justify the freeze (which is bringing some quietness and less distraction to the releng duties for those to be handled as best).
But if the requirement is " a complete SDK build including some extra commits you'd like to test", then I think we can uncouple it from the build infra, and instead evaluate possible approaches to more easily build a custom SDK, or patch an existing one, with locally built bundles. I'm don't know yet what a technical solution could be, but as mentioned, I think it shouldn't rely on the freeze/release/I-Build process for that; it'd better be something easy enough to do on user's end.