My friend who works on the MAME project told me that it's a very friendly platform for adding new emulators (whether you want to add an emulation of a game, a device, a platform, or whatever) because it provides a good structure to work with and a lot of tools to describe recurring patterns in electronics and systems. That might be a good middle ground to start with -- trying to add support in MAME for some device that's not presently emulated there. (Apparently the MAME project really appreciates such contributions, since they think of themselves as pursuing mainly a historical preservation and documentation mission, and would always like to see more devices covered.)

Although that's not "from scratch" because it would still be using their libraries and plumbing, it could be "from scratch" in the sense that you might take an environment with 0% support for emulating a certain device or system and build it up to having 100% support. So it seems like a nice way to start.