I've only done hobbyist embedded development, so maybe I'm totally off the mark here, but I would have guessed that for truly low RAM niches (i.e. ultra low pow

I've only done hobbyist embedded development, so maybe I'm totally off the mark here, but I would have guessed that for truly low RAM niches (i.e. ultra low power or extremely cost constrained), an RTOS would be totally out of the question. And for the vendor-provided libraries, if you're really memory constrained, you're probably writing in assembly language anyway which dwarfs any other porting cost.