Peter's solution is nice. I revised his XML to be consistent with my original
post:

<Document>
    <AIRCRAFT xml:id="AIRCRAFT" ref="ALTITUDE"/>
    <ALTITUDE xml:id="ALTITUDE" ref="UNITS"/>
    <UNITS xml:id="UNITS">FLIGHT_LEVEL</UNITS>
</Document>

AIRCRAFT references ALTITUDE which references UNITS which has value
FLIGHT_LEVEL.

The below XSLT traverses the reference chain:

<xsl:template match="/*">
    <xsl:message>
        <xsl:value-of select="AIRCRAFT/id(@ref)/id(@ref)/text()"/>
    </xsl:message>
</xsl:template>

which prints:

FLIGHT_LEVEL

Thank you, Peter.

/Roger