Sorry this is so late, but the original thread quickly (and not
unhelpfully) went the way of what things should look like in P5. I'm
hoping to address the original poster's problem in P4.

> <abbr expan="mannum" type="">mann $,1!+ (B</abbr>


> I then realized that this way it was going to be very difficult
> to follow editorial practice (highlighting expanded letters using
> italics) ...

True.


> ... and that it will require a considerable effort to update my
> transcription to the forthcoming TEI P5 standard.

I think this is blatantly false. I honestly believe that transforming
proper P4 practices like the above into P5 will not only be
automatable, but most of it (including encodings like the above) will
be trivial. This isn't to say I couldn't be wrong, but I'm perfectly
willing to bet my project's future on it.


> mann<abbr> $,1!+ (B</abbr><expan>um</expan>

Not at all unreasonable, but why two elements? In the P4 world, seems
to me this should be encoded
   mann<abbr expan="um"> $,1!+ (B</abbr>
Unless you have non-Unicode characters to deal with in the expansion
and are using P4 XML, this should work just fine. An old article on
how this can be helpful in dealing with multiple editorial
interventions in a single word can be found at
http://www.wwp.brown.edu/project/newsletter/vol02num02/challenges022.html


> I can now write a simple style sheet according to the kind of
> diplomatic edition I want to output from my text, keeping the
> content of <abbr> "as is" for a conservative edition, or printing
> the content of <expan> in italics for a more interpretative one.

Same holds, except now it's the value of expan= instead of the
content of <expan>.


> Furthermore, I will be able to make my document P5 compliant with
> a simple search and replace of <abbr> and </expan>.

This is quite reasonable if you know your data thoroughly, and are
confident there are no uses of either element without the other (and
no occurrences of those strings in comments, PIs, CDATA marked
sections, etc.). But as I said above, transforming <abbr> with expan=
into whatever P5 uses should be trivial.