The ATF handles topics of "permanently attached" much like they do with firearms design approvals and what constituted "destroyed" - they just test and think about it and change their mind at-will, if they don't think their approved design/process is sufficient for the spirit/intent of the law. Currently "permanently attached" is rather clearly defined and "industry standard" does not really deviate in any way. That is to say, if you deviate, you'll be sticking out like a sore thumb and won't have a leg to stand on.
http://www.atf.gov/files/publications/d ... pter-2.pdf
Section 2.1.3 reads:
The ATF procedure for measuring barrel length is to measure from the closed bolt (or breech-face) to
the furthermost end of the barrel or permanently attached muzzle device. Permanent methods of
attachment include full-fusion gas or electric steel-seam welding, high-temperature (1100°F) silver
soldering, or blind pinning with the pin head welded over. Barrels are measured by inserting a dowel rod
into the barrel until the rod stops against the bolt or breech-face. The rod is then marked at the
furthermost end of the barrel or permanently attached muzzle device, withdrawn from the barrel, and
A blind-pin that interferes with the threads of the barrel, with a weld covering it up, is the most common, I believe. It's also the least amount of energy imparted to the barrel post-manufacturing, as well as being quite easy and quick to do, and clean up after the fact. It is also the most easily reversed in case you wish to have a gunsmith install a different muzzle brake.
ETA: Going from an RRA heavy to a DD lite is an upgrade, not a downgrade.
You can easily source 14.5" barrels with a permanently attached MD. Bravo Company comes to mind immediately, but there are others I'm sure.