Meanwhile, because of the current thinking in the medical field, the person whose need is the most immediate gets the first call, even if they may have no chance of surviving
That is totally not correct. The person with a poor prognosis is not even put on the transplant list in the first place.
Similairly, in most overcrowded ICUs, those with a poor prognosis will get transfered to a regular unit with less life-saving interventions available, so that those with a better chance will get the better ICU care.
I DO NOT know the halachic perspecitve on this issue, and I would be very interested to know.