A quick correction to my previous message and an apology - I had quickly typed up my proposal during a quick break at work, without actually testing them on real Tails 3.2 rebuild - I'll be sure to double check any proposals (and obviously test any proposed changes to the live system with a vagrant build) beforehand in future.
Option 3 (by rebuilding the Tails squashfs) does work perfectly, as does a hypothetical 'option 4', which includes a preferences/torbutton.js instead of prefs.js. In the former case, Tails defaults to Torbutton 'High' settings only at boot, in the latter case, every time tor-browser is closed and reopened.
Options 1 and 2 using dotfiles persistence, however, do not (yet) work as intended. The .js files are accessible within .tor-browser/... as symlinks to /live/... as expected, but Tor Browser itself is unable to read, parse, or write to them. They are fully operational from the point of view of the amnesia user's bash shell though. I'll investigate this further.
Personally, I still feel that option 3, which defaults Torbutton to high at each boot, whether persistence is enabled or not, would be the most security-conscious option, while not getting too much in a user's way should he wish to enable JS. If intrigeri and his team would rather keep JS opt-out rather than opt-in, I understand, and work on a documentation-based solution for 'advanced users' instead.
