Compose key on external blutooth-connected keyboard
The nearest I could find to a solution for this question was found at another Stack Exchange question.  According to the "Keyboard layout" subsection of the User's Guide for Hacker's Keyboard there is a compose key available in their keyboard app.  The thing that I'm not yet able to determine is whether that functionality can then be mapped to a key on an external keyboard.

Unless some other user beats me to a better answer, I will do further research and try to update this answer with any new information I should happen to discover.