Solving this is tricky if only because of the complexity of the DTD design and the fact that few of us write DTDs every day. The fact that there is quite a bit of stylesheet machinery around termref/xtermref/termdef means we need to be cautious.

I'm inclined to the following rather clunky solution:

(a) add an attribute <termdef open="true"> to indicate that the termdef continues beyond the end of the actual termdef element;

(b) add the markup <p role="closetermdef"/> to signal the end of the most recent open termdef.

The only stylesheet changes are then, I think

(i) Change or override the match="termdef" template (xmlspec line 2145), currently

  <xsl:template match="termdef">
    <xsl:text>[</xsl:text>
    <a name="{@id}" id="{@id}" title="{@term}">
      <xsl:text>Definition</xsl:text>
    </a>
    <xsl:text>: </xsl:text>
    <xsl:apply-templates/>
    <xsl:text>]</xsl:text>
  </xsl:template>

to omit the closing "]" if open="true"

(ii) Add a template rule

<xsl:template match="p[@role='closetermdef']">
  <p>]</p>
</xsl:template>

And of course we have to change the stylesheet code that produces the glossary (xsl-query.xsl line 546). I'm not sure about the diff case, but the non-diff code replaces

        <dd>
          <p>
            <xsl:apply-templates/>
          </p>
        </dd>

by 
        <dd>
          <p>
            <xsl:apply-templates/>
          </p>
          <xsl:if test="@open='true'">
            <xsl:variable name="close" select="../p[@role='closetermdef'][1]"/>
            <xsl:apply-templates select="../following-sibling::*[$close >> .]"/>
          </xsl:if>
        </dd>

I have provisionally made (but not committed) the suggested DTD and stylesheet changes, and used them for four definitions in the XSLT spec that logically ran into multiple paragraphs.

I found that some of the templates needing change were overridden in the XSLT version of the stylesheets, so the changes to the base templates have not actually been tested by this process.

(In the course of this I noticed that oXygen, which I use for authoring the specs, was actually using its own version of xmlspec to validate the documents, rather than using the version that we link to. So changing the DTD didn't clear the apparent validation errors. I've changed my oXygen configuration to prevent this happening. But the serialization spec still shows up as DTD-invalid.)

This was discussed between the editors, and it was decided not to make any changes to the serialization spec.