Interesting question. My initial reaction was that the past tense of "plead" is "plead." As in, "I plead guilty." But then I thought of the case as in "was formerly pleading," and in that case I feel like it should be "pleaded," as in, "I pleaded for his life but they shot him anyway." But I guess I would be okay with "plead" in both cases, but I would definitely not be okay with "pleaded guilty" which sounds like "bestest friend" to me.

Somewhat related, I am often bothered by "obligated" instead of "obliged" and similar words.