I used all upper-case element and attribute names in XML: A Primer (and do so
generally) because it's a lot easier to separate markup from content visually
that way. Mixed case is more attractive if your document is really about the
markup, but I tend to be more interested in the content.
Since I'm one of those neanderthals who still does HTML and XML development by
hand in a text editor (or in source code), using uppercase is much handier.
It won't matter nearly as much to me when I find a program that I actually
_like_ to use for markup creation.
Some programs use all lower-case; I find that it doesn't stand out nearly as
well for me. I don't think anyone would have a problem with your proposal;
just make sure you make clear to all participants that _case matters_ when
starting out on your DTD design process.
Simon St.Laurent
Dynamic HTML: A Primer / XML: A Primer / Cookies