For anyone that may be interested, I spent some time working on a version of MicroMonitor (http://www.umonfw.com) called uMon-M (uMon for MCUs) to run on these Cortex-M3/4 type devices. Its basically a trimmed down version of MicroMonitor with networking and a basic flash file system that overlays a configured portion of the internal flash. I have it running on the K64FRDM board (and a few others) if anyone is interested in trying it out, it would be great to get some folks using it and beating on it a bit. The tarball uses some code from the SDK, and also includes a few example applications that demonstrate how a bootloader/application configuration could be set up.
Its still being "tuned", trying to cut it down in size as much as possible; but still maintain a core set of features useful in a bootloader environment.
Currently it builds for two different modes, primarily just for footprint size. The small footprint supports a downloadable network node (ARP/ICMP/TFTP) and some configurability for bootup. The larger footprint supports essentially the same stuff (with a lot of fat removed) that regular uMon supports.
There's a "getting started" document here: http://www.umonfw.com/docs/uMonM_GettingStarted_K64FRDM.pdf
If interested, reply to this or just download it and give it a shot. I'd love to get some feedback on its usefulness in these types of systems.