Please pardon my asking a (probably) stupid question, but what does a
horizontal list look like? Could it just be a part of a normal paragraph
where something is listed, e.g.
... (using \vbox or \vtop or \vcenter or \valign or \vadjust or \insert).
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)?
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 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.