Hello guys !
I'm a fresh new member of this Fandom, but I've found something that I didn't see anyone talking about it before : there is a clue about Libitina in the game files. That's. Right.

Alright ! Let's talk a little bit informatic first ! Thanks to a video (link : https://www.youtube.com/watch?v=qflpAI2T6mI), I dearchived and decompiled the game files, like when you have to do a mod. Then, because I'm curious, I've decided to explore the game files. And particularly the poems.rpy file. That's the file that has the text for the poems.

Now, let's talk gameplay a few moments, then I'll get back to source. At a moment of the gameplay (sorry, forgot when --'), Yuri gives you beautiful poem, filled with... blood. This poem is, at first very creepy but most important of all, incomprehensible (see image). BUT ! Thanks to the informatic decompilation, I could have access to it...

We're now getting back to the file ! The poem that I'm talking about is named "poem_y23", in the poem.rpy file. At first, there's a lot, a LOT of random words : "ed, zinger suivante, tels handknits finish"... BUT ! There is also, and that's the part you were asking since the beginning, a clue, for the now well-known "Project Libitina". And thanks to THIS specific poem, I can develop a deep theory, about Libitina's name.
After hundreds and hundreds random words, this text is shown (read carefully, it's important ;-) ) :

"Fresh blood seeps through the line parting her skin and slowly colors her breast red. I begin to hyperventilate as my compulsion grows. The images won’t go away. Images of me driving the knife into her flesh continuously, fucking her body with the blade, making a mess of her. My head starts going crazy as my thoughts start to return. Shooting pain assaults my mind along with my thoughts. This is disgusting. Absolutely disgusting. How could I ever let myself think these things? But it’s unmistakable. The lust continues to linger through my veins. An ache in my muscles stems from the unreleased tension experienced by my entire body. Her Third Eye is drawing me closer."

You read all of it ? Nice.

Now as you could think, I'm going to analyse this. Obviously, here, it's Monika who's talking. But what's she doing ? Eh ? She's killing someone. And how you may say ? With a knife. A KNIFE. And who's the ONLY girl who's killed by a knife ? You're right, that's Yuri. So here, Monika's killing Yuri, with a knife.

Now we understand that, we can continue to our last part, the very last sentence, "Her Third Eye is drawing me closer.". Project Libitina's page is also referencing to this Third Eye. So now, since we know that Monika's talking to Yuri, she speaks about Yuri in the last sentence, because she is referenced by "Her". SO ! Monika's talking about Yuri's Third Eye !

So, can we confirm now that Yuri is Project Libitina's patient ?
I think we can.

I hope this post was interesting for you to read (even if it was long ;-) ),
Thank you for reading,
Newend.
(P.S : Goddamn that was epic X'-D )
(P.S n°2 : sorry if there's some language mistakes, I'm French ;-) )