I know the Rek'Sai teaser you're talking about (with the Noxian explorer's journal) but if you remember, he did not successfully capture a Xer'Sai to send back to Noxus. Instead, even small ones were able to kill accomplished trackers and beasts.
As for Rek'Sai herself? Oh dear me no - if the Noxians can't even capture one of her children (small and weak and stupid in comparison) they have NO HOPE WHATSOEVER of capturing Rek'Sai. The idea of being able to then use her as some kind of weapon... wow. That's an ambitious thought.
Maybe I should turn it around - what do you think it would take to be able to face Rek'Sai her preferred territory, the Shuriman Desert (The Sai'Khaleek)? How many soldiers do you think you'd need, or something like it. I'm not asking about in-game. Rek'Sai can burrow through sand and solid rock, travels faster than most land-animals, and is many times bigger than a camel.
(Felt I could also link her color story: Sai Kahleek, which follows someone who ventures into the desert.)