> Please pardon my asking a (probably) stupid question, but what does a
 > horizontal list look like?

not a stupid question. in most cases you see only enumeration lists as
horzontal list as in

 consider the options a) a \vbox b) an \hbox c) \vcenter which can all be
 applied at this point ...

 > or is it understood that there is some clear separation between the items,
 > like in the TeXbook's example of footnotes-in-a-paragraph (p. 398, at least
 > in my copy)?

that would be another example.

sometimes you also find itemize like lists of this type (typically in case of
sublist, ie not really running within a normal paragraph text.

 > PS: I've given some thought to the footnote template type, and so far I
 > haven't been able to come up with anything that would require more than the
 > single argument for the footnote text. (A footnote document commands might

well, current latex already has two for \footnotetext (one being the text and
one being the number to use or \NoValue)

i'm not really into critical edition type of footnote handling but perhaps
there are things that might be helpful to provide

 > well take more than one argument---it could for example take an argument
 > specifying who's comment this is, in the case of footnotes being used for
 > commentry to a fixed text---but that would rather be used to select the
 > right template instance.)

 > On the other hand, there are so many
 > possibilities for what should happen to the argument that I doubt you'll
 > get by with one (or even two) base templates.

in that case please think about specs for the base templates (not necessarily
the implementations though that is fun as well) that you can think would be
sensible, ie what should they do (the templates) or more exactly what layout
should they provide and what could be possible attributes in each case.

i have in fact gone half through this excerise last night and implemented a
few rough drafts and i intend to show them for discussion end of the month as
i will get away for a short vacation tomorrow at 4:00

in the meantime thoughts on such base templates and their attributes would be
welcome (i didn't got very far last night)