That's incredibly exciting! Congrats! and Happy early birthday!!
It's unfortunately unlikely for the Rocko character to still be in Universal Orlando's possession. Even if they really wanted to treat you to a special encounter, I believe they are unable to since the costume just isn't around anymore.
I might be wrong but if what I've heard is true, I believe that during HHN hours at USF, the extended park areas that lead to the Soundstage houses allow you to utilize the Blue Man Group bathrooms, which feature effectively the same green splatter theming as when it was the Nick Studios. So that could be a fun thing to try and check out!
If HHN is out of the question, then the back pathway behind Hard Rock is the current closest spot to scope out the former Studios building! A lot of it was visible from the HRRR queue, but with that down, most of that area is a big construction zone currently!
Not sure how you feel about these Nick characters, but you can also try and meet Spongebob and the others at his store, and they might have a fun reaction to seeing your old-school Rocko!
There's also some fun references to T-2 in the current Bourne show, which is one of my favorites across the whole resort!
I recommend the single-riders queue for Doom! Unless something's changed since I last tried it, it should be available through the casino arcade next door and is a super easy way to loop that ride over and over with very little wait!
Congrats again! I hope you have a wonderful vacation!