storing a directory name with spaces in a bash variable
OK, found this by broadening by googling without specifying mac or osx - this is a generic bash issue:  Stackoverflow how to add path with space in bash variable

And, translating that to my question, both "interactive approaches", i.e. quoting or \-escaping work, as long as you wrap the bash variable in its own double-quotes when you run the commands:

dn='/Users/myuser/Library/Application Support/'
ls -1 "$dn" | tail -5

dn=/Users/myuser/Library/Application\ Support/
ls -1 "$dn" | tail -5


Don't use single quotes above - there is no bash substitution happening in that case and you get ls: $dn: No such file or directory.

The drawback here is having to wrap the variables in double-quotes or not depending on embedded spaces.

dn="'/Users/myuser/Library/Application Support/'" and then not having quotes doesn't work, btw.