As far as I know the AsciiDoc variables that are available and defined in the pom are set in:
build > plugins > plugin (node corresponding to asciidoctor-maven-plugin) > configuration > attributes (see
Lines 47-49 of in the to-html example pom)
or
build > plugins > plugin (node corresponding to asciidoctor-maven-plugin) > executions > execution > configuration > attributes (see
Lines 112-119 of in the to-html example pom)
Have you tried to write something like that?
<attributes>
<blah.blah>${some.maven.property}</blah.blah>
<!-- ... -->
</attributes>