Font problem - need 2 font files in Docbook

Am I missing something or is this a real restriction in Docbook

In text body I have two symbols which (as far as I can see) never occur 
in the same font file - specifically the old english yogh (ȝ) and 
the logical symbol implies (⇒ ISOtech suggests using this for 
'implies').  (Gentium say that eventually they will both be included, 
but  not yet.)

Using XEP 4.18, in my xep.xml file I have:

       <font-family name="Symbol">
         <font><font-data afm="Symbol.afm"/></font>
       </font-family>

which contains 'implies' (&#x21D2;) and is built in to Acrobat and

       <font-group xml:base="file:/Library/Fonts/" label="TrueType" 
embed="true">
       <font-family name="Cardo">
         <font><font-data ttf="Cardo98s.ttf"/></font>
       </font-family>

which I embed in the PDF output and which contains 'yogh' (&#x021D;)

In my Docbook customization file I have:

<xsl:param name="body.font.family" select="'Cardo'" />
<xsl:param name="body.font.size" select="'11pt'" />
<xsl:param name="title.font.family" select="'Cardo'" />
<xsl:param name="footnote.font.size" select="'90%'" />
<xsl:param name="footnote.font.family" select="'Cardo'" />
<xsl:param name="alignment" select="'justify'" />

in the PDF output 'yogh' appears correctly, but 'implies' does not.

If I reset body.font.family to the Docbook default (eliminating the 
above customization) I get the 'implies' (&#x21D2;) correctly, but lose 
the 'yogh' (&#x021D;)

I have entities defined and know that the entity file is accessed.

<!ENTITY  yogh "&#x021D;">    <!--   lower case yogh -->
<!ENTITY  rArr "&#x21D2;">    <!-- rightwards double arrow -->

I cannot see any way in Docbook to change the body.font.family for a 
single body character, or to specify a secondary body.font.family to be 
used if a character is not found in the first one.


-- 
Ron Catterall Ph.D. D.Sc.

http://catterall.net