Well, looks like there is a balance between time and money here. For example, instead of removing the stringers altogether it seems as though the following will work:
(1) Cut the tops of the shell off the stringers (just enough to get to the wood).
(2) Pull out any soft rot.
(3) Use CPES (
www.rotdoctor.com) for any of the wood that remains inside the shell - anything that wasn't fully rotten.
(4) Use seacast to fill the shell wherever it isn't fully filled with the wood (ie: where you pulled the rot out of).
One assumption: seacast will stick to CPES. It is an epoxy, so I don't see why not. Seacast doesn't stick to regular wood so well (according to the seacast site), hence the need for CPES.
The cost of this is probably higher than clearing out the old stringers and putting in new ones. On the other hand, you avoid all that grinding and much of the fiberglass work, so it seems like it will save time on that.