Tristan McLeay wrote:
> > the rule could be expressed in terms of kanji, e.g.,
> > ha/hi/fu/he/ho =
> > wa/i/u/e/o in the middle of a kanji and in
> > inflections.
> I'm a little confused by this.

What I mean (probably poorly phrased) is that, for example, the furigana
over the "river" kanji was _kaha_, and since _ha_ is in the middle of a
group of kana over a single kanji, you know it's pronounced _wa_ rather
than _ha_.  On the other hand, in _nihon_, you have _ni_ over one kanji,
and _hon_ over the next, thus, _ho_ is at the beginning of the kanji's
furigana, and is, therefore, pronounced _ho_, not _o_.

However, pure kana in the historic orthography would be a little
trickier.  Even if you added word spaces, you'd have to know the origin
of words to know that _nihon_ was /nihon/ rather than /nion/, while
_kaha_ was /kawa/ rather than /kaha/

Incidentally, _hawa_ for "mother" is attested in some Tokugawa-era
novels and Noh plays.  I've read a theory that _haha_ was re-created by
analogy with _chichi_, which uses reduplication.  Of course, the most
amusing thing about that word is that /h/ comes from /P/ which in turn
comes from /p/, so the Old Japanese called their mothers "Papa"!  :-)
There's an old riddle from the Heian era (794-1160) which is slightly
ambiguous (the key to the riddle).  The normal interpretation would be
"What meets its mother twice, but never its father?  The lips".  The key
to the riddle is that _haha ni_ can be a location as well as an object,
thus "What meets [itself] twice in 'mother', but never in 'father'?  The
lips".  The answer only makes sense when you realize that alternate
interpretation, and know that "mother" was [papa] at one point.  :-)

> Given the phoneticity of the modern orthography, it
> seems amazing that they'd keep something around for 11
> centuries...

Well, for much of that time, the various _wo_ kana were used
interchangeably with _o_ (since there were already so many alternates
for kana, it was easy for formerly distinct kana to be reinterpreted as
alternate forms of the same kana), it was only sometime during the
Tokugawa era (17th to 19th century) that the distinction was
reintroduced, based on older spellings.