I'm a painter by trade and my specialty is wood finishing. I know that linseed oil allows the wood to breathe, but not sure if it would be toxic if used on a pipe. There is a coating called Penofin that has a hardwood finish that supposedly allows wood to breathe. It is used on hardwood floors and such but again I don't know if it would be toxic or not. Here is a paragraph from there website:
Only Penofin’s Hardwood finisH can penetrate dense hardwoods that need nourishing and stabilizing. This unique formula
penetrates deep into the wood fibers and does not create a surface film. Unlike high-solid stains that simply coat the surface, Penofin’s
Hardwood Finish, using sustainably harvested Brazilian Rosewood Oil, is the choice of wood experts who know that wood penetration
is key to longevity and beauty. Penofin allows wood to breathe, important in working with exotic hardwoods such as mahogany,
teak, Ipe, Brazillian Redwood, and many other species. Only Penofin Penetrating Oil Finish can meet the needs of hardwood in fiber
protection, penetration, stabilizing, and nourishing.
Don't know if this matters really, but it is a little informative!!!