Being allergic to nickel must be a real problem for a guitar player. You can't buy anything off the shelf.
Congratulations though, that is a sweet guitar!
Tried this method out with different guitars, I'd have no problem playing a Stratocaster and getting the "gain" I want to hear. Remarkable how this can make such a difference.
The wood and the truss rod should be fine leaving under tension.
That being said, if you are traveling with a guitar, it would be wise to loosen / detune your guitar to reduce any added pressure if there is a bump at the headstock area.
Tried this last night. Gain down, volume up, reduction of tone controls. What an amazing sound I got. I was rather pleased by what I heard. Sounded much better.