How to pass XSLT params to ant.xslt() task?

Ant has method naming conventions and will call setX(...), createX(...) and addX(...) methods on the task based on the xml config. See Writing Your Own Task.

I believe your snippet would translate to

ant.taskdef(name: 'xslt', ...)
ant.xslt(basedir:"doc" destdir:"build/doc" extension:".html" style:"style/apache.xsl") {
    param(name:"date" expression:"07-01-2000") 
}