If it is done, then it's VERY important that it be non-stackable so as to keep pyromaniacs from having a regen rate comparable to Pei. And it should go without saying that the boost should be active only while sitting, and weak enough to not be a substitute for Invulnerability. I'm sure there's other "creative" uses of such an addition, but those are the ones that come to mind immediately.
Overall, I like the idea, but have reservations about the implementation.
Yeah clearly, only usable when sit and not stackable, also the regen rate should not be craycray, something like +2 or +3 on the 4 main's stats overall.
Watching fire can warm you, make you focus better, and ease and calm your mind (spirit/sap) etc, the roleplay reason could be within this.
If its done like this, +1 for me :)