Sam: I still must disagree with you from both a Halachic and practical perspective.
Furthermore, your definition of Eiva is excessively broad. All of the cases mentioned in Shas and Poskim are where the Yid is refusing to assist a Goy purely due to the fact that he is not Jewish. We only find concern of Eiva when confronted with this blatant discrimination; however, here we are not suggesting that one donate his organs to Jews and not Goyim. When a Jew declines to donate his organs in general to anyone for religious reasons, there is no indication that a concern of Eiva exists. In fact, Rashi Bava Metzia 32b s.v. ?? ???? ??? ???????? clearly states that when one can explain that he cannot assist the non-Jew to load his donkey with Yayin Nesech for religious reasons, Eiva no longer applies. This is cited in the Beis Yosef CM 272.
In fact, it is clear from the Gemara Avoda Zara 26a and Shulchan Aruch OC 330:2 that one may even discriminate between Jews and non-Jews when a credible excuse is given.
Additionally, we only find Eiva when a Yid refuses a direct request for assistance. Merely failing to sign an organ donor card is not comparable.
On the practical angle, I have not seen an organ donor card that allows one to specify that one ONLY wishes to donate after cardiac death. Most do not differentiate at all, and once one has signed there is no guarantee that they will not murder the brain-dead signatory to use his organs.
In fact, the secular world considers brain death the more reliable indicator of the cessation of life. Many states and hospitals do not accept cardiac death from their moral perspective and will refuse to remove organs from a patient who has suffered cardiac arrest. They would understand any stipulation of cardiac death in a living will as including EVEN an individual who has merely undergone cardiac death, and not as excluding a brain dead person.
By way of illustration, less than 5% of deaths are brain deaths, but they comprise 92% of organ donations. Most US hospitals cannot, or will not, accept organs from one who has suffered cardiac death. They will ONLY accept organs from a brain dead, but Halachically alive, patient.
In conclusion, anyone who signs consent to donate his organs is asking to be murdered, and there is no Heter due to Eiva even when truly dead.