Is it valid to use "for good" to mean for the sake of good, as opposed to forever, as in "Doctors for Good" for example? How would one edit the phrase to indicate altruism without replacing the word "Good"?
It would be more natural to say "for the sake of the good." But that still sounds awkward and I would recast the sentence.
"For good" in the sense of forever is a very informal expression. Your example, "Doctors for good" is not at all natural-sounding. A better example would be: "So you're giving up smoking for good?"
The sense you are seeking would generally be cast either as "for -the- good", as in:
"he acted for the good of humanity",
or placed in a context which clearly denotes "good" as something beneficent, rather than as an indicator of time --for example:
"Superman decided to use his great powers for good, rather than for evil."
(Indeed, some dictionaries define "beneficent" as "generous, or doing good".)
"Doctors for Good" would probably be better cast as "Doctors for the Good", or "Doctors for the good of humanity", or something similar.