> I tried that but ran into trouble, I think because of the INHERITED
> parameter entity required by a.global.

If you redefine something within the DTD subset, this definition is read
before the rest of the DTD, and so cannot contain any parameter entities
defined later within it. Therefore, if you are doing this within the DTD
subset, you must give the "full" version of your declaration, thus:

<!DOCTYPE TEI.2 SYSTEM "tei2.dtd" [
<!ENTITY % a.global '
      href CDATA #IMPLIED
      corresp IDREFS #IMPLIED
      synch IDREFS #IMPLIED
      sameAs IDREF #IMPLIED
      copyOf IDREF #IMPLIED
      next IDREF #IMPLIED
      prev IDREF #IMPLIED
      exclude IDREFS #IMPLIED
      select IDREFS #IMPLIED
      ana IDREFS #IMPLIED
      lang IDREF #IMPLIED
      rend CDATA #IMPLIED'>
<!ENTITY % TEI.prose 'INCLUDE'>
<!ENTITY % TEI.linking 'INCLUDE'>
<!ENTITY % TEI.figures 'INCLUDE'>
<!ENTITY % TEI.analysis 'INCLUDE'>
<!ENTITY % TEI.XML 'INCLUDE'>
]>

This assumes you are using the analysis and linking, but not the terminology
classes. If you are not, remove/add the relevant attributes (see
http://www.tei-c.org/P4X/ref-CLANA.html,
http://www.tei-c.org/P4X/ref-CLLINK.html,
http://www.tei-c.org/P4X/ref-CLTERM.html).

Even if you are making this amendment in your local modifications file
pointed to from TEI.extensions.ent, you still need to do this. In fact it is
worth noting as a general point that any modifications to TEI parameter
entities cannot reference other TEI parameter entities.

Hope this clarifies.

Stuart