According to the classic Hethitisches Zeichenlexikon, Hurrian used a handful of special signs made by combining the WA (aka PI) sign with others: wa+ap, wa+i, wa+pí, wa+ú, wa+e, wa+u, wa+pu, wa+ip, wa+a. (HZL transliterates them u̯aap etc.)
The authors have this to say about those (p253, my translation):
For the sign u̯aa appearing in Proto-Hattic, Hurrian, and Palaic word-forms and texts, only the traditional transliteration is given here. Due to the still-uncertain state of the art, a phonetic/phonological interpretation (like fa?) is avoided. […] Palaic spellings of this sort likely come from Proto-Hattic influence […] The exact Lautwert must be determined in individual languages.
But this was published in 1989, and I haven't been able to find any more modern discussion of these signs. Has any consensus been reached since then? (And as a side note, what is this "Proto-Hattic" (protohattischen) they're talking about?)