I've submitted ticket #1600 for this request.
But just to make sure you know, it is quite reasonable to store the
<taxonomy> in an external file and then either
a) XInclude it into your <teiHeader>,
b) point directly to the external file (e.g.,
c) point directly to the external file using a private scheme URI
mapped out by a <prefixDef> in the <teiHeader> (e.g.,
Choice (c) is the easiest to encode, and the most robust & flexible;
but it is also a bit harder to set up and significantly harder to
write code to process. Choice (a) is only better than putting it in
your <teiHeader> by 1 step. (But at my project, that one step -- that
the people doing the encoding shouldn't have the burden of seeing or
dealing with the <taxonomy> -- is what we care about, so we use
choice (a) for now.) My sense is that most projects lean towards (b),
but that is based on scant and ill-recalled evidence of people who
were *planning* to use <taxonomy> in their projects, not actually
doing so much yet. :-)
And, of course, the same three possibilities could be used for your
prosopographic data, too.
> I would prefer to keep the teiHeader simple and avoid taxonomies
> because we are dealing winth a lot of prosopographic entries.
> Said that, can anyone tell me more about the reasoning behind the @type
> attribute? I still think it would be the best approach.
 See https://github.com/TEIC/TEI/issues/1600. If you have a
GitHub account (and maybe if not, idunno) you can arrange to get
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