The question you have to ask yourself is "What the

does an Ork genestealer kult leader look like?"
My suggestion for rules:
Require a broodlord in the Tyranid army, or a genestealer cult leader if you can find stats and points for it. If you can figure out a decent warboss with carnifex or genestealer bits, conversion, do it.
If the Tyranids are winning on the third turn, they have to start eating the orks. Or better yet, if Tyranids and Orks end up in a mixed assault, require an leadership test (ignoring the psynapse rules, no fair trying to get out of rolling) for the Tyranid unit to avoid attacking the ork unit that turn.
It's
Apoc, it's okay if it's a bit silly, right?