Raspbian 2017-04-10 USB Boot

Are there any SD card builds in the works for a USB-only image based on the latest Raspbian from April 10?

Not that I know of. You will probably have to make a Raspbian image, then add emoncms etc according to the SD Card build guide.

Before I posted this question, I began with that, but stopped at the Read-Only config, realizing that may be specific to SD cards. I considered skipping that step, but noticed a note about file system type necessary for MySQL.

I can return to that. Should I still proceed with the Read-Only partition step? Or is that unnecessary?

The read-only OS is there for 2 reasons, firstly to prevent excessive writes reducing SD card life and for protecting against accidental deletions and edits etc on a system that once set up, is just left ticking over without much change.

If you are setting up a non-SDcard server there is no concern over excessive writes so the decision is entirely up to you and whether you are doing other stuff eg a desktop and/or if you can trust yourself (or others) not to mess something up accidentally, the RO option doesn’t eliminate the latter but it makes you think about what you are doing when you have to select read and write mode. Personally I no longer bother with RO on pi+hdd servers.