A rudder should be considered an "easy" repair since it can be removed from the boat and taken indoors to a workshop. (esp outboard rudders on smaller boats).
In a workshop you can more easily remove all the rotted interior and dry the whole rudder out thoroughly. I have a friend that did this process two years ago. He removed all suspect material and then used West System (he is a big fan) to refill the affacted areas. He then rolled on an
epoxy coatingr (again West System) over entire rudder prior to painting. Email me at mike.hoyt@impgroup.com and I can put you in contact with him for full details.
I also agree that injecting is less secure than removing the rot. I would inject
epoxy in small problem areas on a deck or cockpit floor repair because it is a whole lot of work to remove rot in thise areas. For a rudder it is much easier to access and to repair properly.
This season I have removed my rudder for inspection and possible repair. Even if I find no rot I will likely coat teh entire rudder with
epoxy anyway.
Best of luck
Mike