Can Reformed Slavic Pagans Request a Great Holy War

Such a vast empire is bound to get split at some point, particularly after succession. You can either force it by causing.. an unfortunate incident, as Kyle suggested, or sowing dissent with a powerful king-level or powerful duke vassal of HRE who dislike their current liege.

Of note is that you can beat larger armies than yours with some clever warfare tactics. You are a pagan, and if the enemy hasn't researched Military Organization at least two or three times (unlikely, if you are that early in the game), then they suffer severe attrition in your territories. Also important is that hauling such large armies can take time, and would give you a few precious months of consolidating your troops and destroying some of their armies while they call their Occitan legions. Hopefully your realm is much more compact than theirs, and have ample naval access for quick transport on sea rather than by land.

If you deliver significant damage to them, factions within their empire might grow powerful enough to cause a civil war. This can give you a significant advantage and might cause them to ask for white peace to deal with troublesome vassals. This will give you some ten years of breathing room for you to expand and grow until they are able to strike again.

As you always have a casus belli against them, yielding some lands to them now without a fight might be a good longer-term strategy to reconquer them later when they are embroiled in another conflict -- particularly if you don't hold any of the counties in the duchy.

If you don't have a claim on the title, and you can't usurp it, you might be able to invite a claimant to your court.

To do that, click on the coat of arms for that title to go to the title screen. One of the buttons on will say "Claimants", and this will reveal a new window with a list of everybody who has a claim on that title. Anybody in that list who has a green thumbs up next to their portrait, is willing to join your court if you click on them.

If there are no one willing, you may try to persuade one of those with a red thumbs down to win them over – however, there is no guarantee for success. Figure out why they won't join you.

Either way, if you are successful, the new claimant will join your court in approximately fourteeen days. You will then be able to press his/her claim.

If the title is lower than your own (as it is in your case), the claimant will become your vassal after the war. If the title is equal to yours or higher, the claimant will become independent.