Hi Marco,
(PS for Hans),

Marco wrote:
> \in{figure}[alpha,beta,gamma]
>
> This outputs “figure 1”. What I'd like to have is “figure 1-3”.

The attached quasi-module seems to do it! On my computer, at least. (I
call it 'quasi' because it is really nothing more than code in a file
of its own. No configurability whatsoever, except to the extent that I
tried to comment well, and split things up into functions.) I'd have
written it so that it properly identifies runs like '1.2.1, 1.2.2,
1.2.3', too, but I can't find a function that will turn reference
strings into such a prefixed strings.

I hope it does what you want! Let me know if you want/need any
alterations, or discover bugs.
Sietse

PS @Hans: if figure numbers have prefix segments, is there any
(combination of) helper function that will return e.g. the string
'1.2.3' given the reference string "ref:fig-three"? I found
commands.savedlistprefixednumber(nil, numberinlist), but that is a
wrapper around sections.typesetnumber; and that last function injects
the string into the TeX stream instead of returning it. Is there
another function I haven't found yet that will do this, or are
prefixes implemented print-only at the moment?