You need an editor with some basic APL awareness. There is an APL-mode for Emacs where pressing dot ('.') switches to the APL layout for the next character. It feels very natural to use. I've seen others use a modifier key (like the Super/Windows key) to enter APL characters, which if done as an xmodmap thing, would work in every program. (The programmer I saw do this prefered to do his APL programming in ed(1) - no joke). I assume the Dyalog APL IDE has something similar.