I have the following category structure:

A
|--foo
|--bar
B
|--foo
|--bar
C
|--foo
|--bar
etc.

The structure is not really negotiable.

I need an index that shows all "Foo" entries, but there doesn't appear to be any way to indicate the different instances of "Foo". The category attribute for MTEntries only accepts labels, not IDs.

Worse, the following in a template:

<code>
<MTentries category="Foo">
...etc
</code>

not only doesn't give me all the Foo entries, but it fails altogether. The page builds, but no content is pulled.

Am I overlooking something? It seems like the actual thing I'm trying to do is completely possible, but it's breaking because of the category naming convention.

Okay, so I'd forgotten that MTEntries and MTEntriesWithSubCategories were pretty much completely different things. So:

Up through 3.2, just use MTEntriesWithSubCategories instead, along with the directory format for specifying categories.

If you're using 3.3(+), the above still works, but MTEntriesWithSubCategories is being deprecated in favor of a new "include_subcategories" attribute for the regular MTEntries tag.
(The 3.3 documentation isn't finished at the moment, but it's item #16197 in the 3.3b1 release notes)