Is it Possible to Coat the Subfloor Around a Toilet Flange to Prevent Rot?
As Tester said there shouldn't be any leaks and therefore no rot. Any leak from the wax ring should be repaired immediately. 

However, pressure treated lumber will last longer in a wet environment if you are concerned about overflows, sweating, or other reasons the floor may get wet in that area.

Coating regular wood is not going to stop it from absorbing water on the edge or if it is soaked with water.

If you are laying tile in that area:
They make a liner product called Kerdi and roll on products like HydroBan for use under ceramic tile in wet areas like showers. Ceramic tile is not waterproof by itself.