DOCBOOK: marked section control/was: SUBDOC with SGML DocBook?

I maintain a couple of marked section categories, e.g.

  <!ENTITY % RFC "IGNORE"> <!-- Draft Only -->
  <!-- more like these -->

I want to simplify the process of generating different 
renderings of the document by putting all entities that
control the various marked section into a separate
marks.sgml file that I could generate on the fly, e.g. 
from a Makefile. 

I then wanted to (C speak) "#include" this marks.sgml
file in the actual document, e.g. like

  document.sgml
     <!DOCTYPE Book PUBLIC "-//GNOME//DTD DocBook PNG Variant V1.1//EN"[
       <!ENTITY   Marks       system  "marks.sgml"  >
       <!ENTITY   chap1       system  "chap1.sgml"  >
       <!-- more -->
     ]>        

I seem to be missing something fundamental here. With the
above I can get actual content in the document (&chap1;), but 
there seems no way to place &Marks; properly, so I must be 
trying to use the wrong mechanism.

I can go about using sed/awk from the Makefile, or using
"cat" to put together a valid document, but frankly, I would
prefer to maintain valid SGML documents in CVS, not fragments.
Is there an elegant solution I am overlooking?

 
                                            b.




p.s.: I also experimented with SUBDOC YES 1 in docbook.dcl, but
Jade declines to process such entities. What am I missing?