How do I type the typeset "space" character?
There doesn't seem to be any way to insert it using the default U.S. input source or other normal keyboard layouts:

$ grep -r '&amp;#x2423;' /Volumes/Ukelele\ 2.2.4/System\ Keyboards/
$


Unicode Hex Input is the only keylayout that uses multipliers. If you use Unicode Hex Input, you can insert ␣ by typing 2423 while holding option.

Or create ~/Library/KeyBindings/ and save a property list like this as DefaultKeyBinding.dict:

{
    "~S" = (insertText:, "␣"); // option-shift-s
}


You could also make a new keyboard layout or add characters to the press and hold popovers. I'm currently using an AppleScript (posted in this question) to insert special characters and plain text snippets.