Not to open a can of worms, but...
One could argue that Spock's World brings in the entirety of Doctor Who as an in-universe work of fiction.
I read somewhere that at some point in the DS9-R Taran'atar picks a fight with an Aliens
Alien in the holosuite, so the same applies. And isn't Predator a part of the same universe? Considering the Jem'Hadar are fairly blatant Predator rip-offs this is quite interesting.
I've read that Ishmael had one or two big huge unauthorized crossovers (to stuff I didn't watch so have no knowledge of), too.