I didn't really want to repeat myself, but I have to for the sake of having it on this thread.
To write a reclist, you need the phonology and phonotactics of a language. That's the sounds, and the rules for putting the sounds together. That's all you need. No need to learn the grammar and the sentence structure and the writing system or anything. UTAU is all about the sound.
Simlish is gibberish written based on the phonology and phonotactics of
English. This means that, essentially,
the reclist would be identical to an english reclist. There really is no point in trying to write one. One could argue that the english reclist itself is already in Simlish, because for the most part it IS gibberish syllables arranged according to the rules of English sounds.
That's not to say it's impossible to do anything Simlish with UTAU. You can still make simlish songs and covers, but half of the work is already done for you: You just need to make/use an English UTAU. Here's a page from the Sims wiki with a list of songs.
http://sims.wikia.com/wiki/Songs_in_Simlish