Hi, Chris!
I ran some tests and it seems if you go into Landscape mode on your iPhone and hit the Keyboard button while the Handwritten Messages screen is up, your phone
should default to the keyboard there on out.
It should essentially work as a toggle: Tap to enable Handwritten Messages, and it'll show up each time; Tap to enable the keyboard and it will show up each time.
Let me know if that doesn't work for you!