> Unfortunately, though, \r does not work in an autonumber.
Now that I've actually tried it , it doesn't just not work, it vanishes as soon as you hit [Apply], whether \r or \x09.
Unicode hacks don't work either, such as
\u000d (CR, Carriage Return : ignored)
\u0085 (NEL, Next Line : renders as "?")
\u2028 (LS, Line Separator : renders as "?")
\u2029 (PS, Paragraph Separator : renders as "?")
If you actually want a forced return between the AN and the paratext, you have to type it as part of the paratext.
And it is somewhat unfortunate, because there are cases where you might want it, such as illustration captions
Caption Text String
Your example of a figure number and figure caption on separate lines is easily supported in structured documents. One solution would be to put the figure number in an autonumber but use a prefix consisting of a forced return (entered as \r or \n) as well. If the figure number and caption appear above the graphic, or at the start of some other element, another is to give the containing element a prefix of a space. Since the caption is a paragraph, that space and the caption would be in different paragraphs and could have different paragraph formats.