If I were coding it, I would allow only exactly 1 digit (%.Nt) to simplify the parsing side of things and bound the required buffer size. Without having written it, it's not clear to me whether further restricting the set of supported values would save much code. I will point out, though, that throwing an error during log_line_prefix processing will lead straight to infinite recursion.
I would parse the log_line_prefix when it is set. Then if there is a problem you just log it using whatever format is in effect and don't change the setting. Then the worst that happens is that logs show up in a format log processing isn't prepared to accept.
That being said, I think I fall in the “just start putting more digits in the log” camp, although it is conceivable the counter arguments might be convincing.
