I've seen Saxon Huscarles in crusades when I've played unmodded VI 2.01. Also in my last game, an English crusade passing through HRE land in Burgundy gained three units of Gothic Knights, and that was just because they passed through German land.

I think Ludens is right in that crusades do generate any unit available in the province they pass through, as long as the units are Catholic. This is set in the units file in the field where you specify what kind of situations the unit emerges in (e.g. "Catholic zealots", "Catholic rebellions", "Catholic bandits" etc.) Therefore Orthodox units don't appear in the crusades.

Sometimes trickles of Orthodox units from an Orthodox faction's stacks will join a crusade if the province held by the Orthodox faction is mainly Catholic and has high zeal, and if the units have low loyalty.