[exim] quoting/escaping in map lists

The various list operators introduced a few releases ago (map, reduce,
forany, forall, etc) are very handy. Is there any syntax that quotes
or escapes the separator character in the list?

For example,

${map{a:b:c:d:e}{[$item]}}

I'd like to construct a list where one of the items in the list is the
3-character string "b:c". Is there a way to do it? I've tried
various things like ${quote:b:c}, etc, with no joy.

Obviously, if I were dealing with these literals, it would be easy to
figure out. I'm talking in general terms here, though. I want to
make a list that contains some arbitrary strings, so I can't predict a
good separator in advance.