Its a really nice idea but probably really lots of work :-)
Possibly less then you might think since using NuttX for the bootloader avoids all hardware driver development (unless you have some special driver need). You simply write the bootloader app on top of the existing driver and that app can be as complex as you choose to make it.
And there is a loot of support for a bootloader app. There are file systems to make access to boot code on file systems totally trivial. There are also useful libraries in apps like apps/system/hex2bin that makes loading received .hex files into memory a no brainer.
So I would say that a simple, specialized, minimal NuttX-based bootloader is only a day or so of effort. But, of course, you can make that as complex as you like. A generic, do-all bootloader could cost you months of effort.
