I feel that in the case of many other suicides from the "famed and ethereal" there is some faith-based recognition that "what is true and good will come to light in time, like gold washed down by a silver stream." Or at least I believe that might serve as an impetus, the aboriginal content of "conviction" itself is far more tenuous for me to grasp. You mentioned a "coup" or "manifesto writing", but I think any such recognition is reducible, in part, to how they perceive their own legacy, and the act of suicide itself abstracted into a sign unto itself. Mishima was "Japanese, but not Japanese", something of a "pillar" in which all knowledge rises up from a Platonic fog and establishes itself in a "pure realm of thought-association" (this was put more elegantly by Walter Benjamin). Or to put it more bluntly, they (men such as Mishima, all great men really) laid claim to an idealization of man and became inextricably bound up by it, as if they felt an urgency in what they perceived as decadence and wanted nothing more than to commit to something purer, to submit to "old world mores" and to permanently ingratiate themselves in the minds of "sensitive young men." A "universalizing" impulse if you will.
I think Buddhism is quite different, since those instances you speak of arise from an optimistic worldview, whereas for the cases abovementioned it is only through a pessimism of the cultural world which grants man an internal sense of decree that suicide can be conceived. The latter is, by its nature, committing something "radical", whereas the former is fortified through the manacles of tradition.