DOCBOOK-APPS: head and foot as params in fo/docbook.xsl?

As I read the stylesheets, the header and footer for the fo xsl stylesheets
are printed out by
<xsl:template match="chapter|appendix" mode="running.head.mode">
and
<xsl:template match="*" mode="running.foot.mode">
which basically put various wrappers around variables they declare, called
$head and $foot. I am planning to add custom headers and footers, and it
looks like I'll have to override those templates -- but it seems like if
they're just printing out those variables, they might as well be params, and
I wouldn't have to override anything.

Is there another way to add custom headers and footers? Is there any reason
those variables aren't params?

Thanks,
Jack Cushman