Making a non-ASCII space character visible

[Will Parsons <varro@nodomain.invalid>]

I have a desire to use the Unicode character A007 (FIGURE SPACE) in a
document and to be able to distinguish it visually from a regular ASCII
space.  This seems to be already done in the case of 00A0 (NO-BREAK
SPACE) which appears as an underscore with a distinctive face.

It *looks* like I should be able to do this via the customization
option "Whitespace Display Mappings", but no matter what I do,
attempting to "Apply" the changes results in an error message, "This
field should contain a single character".  (This message occurs even
if I try to change one of the display characters of an existing entry
in the list and apply.)

Feeling desperate, I copied the existing value of
whitespace-display-mappings directly into my custom.el file, and
manually added a new entry for A007 (8199) on the model of the
existing entry for 00A0 (160), but this seems to have no effect on the
display.

(This is using Emacs 25, in case it should make a difference.)

-- 
Will