Plasma donation centers in the US already separate out nothing but plasma and return the rest back to the donor. The plasma is then fractionated for use in medicines so, those types of donations are not "the general blood supply". It seems to me from what I have read of that letter, donating plasma to a plasma center would be allowable. They throw away what they don't use for medicines after extracting the components for medicines. Let THAT get out into congregations of largely financially struggling witnesses! They can put an extra $200+ in their pockets every month donating plasma!
Plasma apheresis donation has become a common practice and is a viable approach for Jehovah's Witnesses. Watchtower policy already allows JWs to accept every bit of plasma so long as it's delivered as cryosupernatant (by volume 99% of plasma) and cryoprecipitate (by volume 1% of plasma). JW patients whose condition would benefit from transfusion of these products line up to accept them.
See:
Jehovah’s Witnesses can donate blood
American Blood Centers' Approach to Jehovah's Witnesses Donating Blood