I don't know J, but an APL keyboard layout is not a problem for any major current OS, nor is a layout with ISO Layer 3 (Mac Option, Windows AltGraph) mnemonically allocated to useful symbols.
Though modern "emoji pickers" have extended the range of IME and soft keyboard tools that regular users use (often daily). I've been joking that the next APL is probably made from emoji, but it's not entirely a joke: the Windows emoji keyboard (Win+. or Win+;) has a pretty full Unicode math symbol section (click the Omega top-level tab and then flip through the bottom level tabs), and while it is missing some of the nice search features of the more colorful emoji, gives relatively quick access to a lot of options.
Is it more demanding than asking them to memorize a collection of (mostly arbitrary) ascii symbols and digraphs? If you learn apl, you have to remember that grade up is ⍋ (s-S-4); if you learn j, you have to remember that grade up is /:. The primary barrier to entry is remembering what operations you can do and how you can do them, not how to type them.
> Requiring users to memorize a second keyboard layout is very demanding
Not really. It's a natural learning process as you learn the language. Very easy. If you are truly learning the language (rather than just messing around) you can get to the point where you are comfortable typing most common symbols in a week or two.
I used to touch-type APL back in the day. Even though I don't use APL at all these days, every so often, when I do, I am amazed by how quickly I remember where the various symbols are on the keyboard. Some of it just make sense, for example "iota" is shift-i, "rho" is shift-r, etc.
