I ran into a new link on Google Books concerning an interesting grammatical sketch of Etruscan supplied in Studies in the Etruscan Loanwords in Latin (1997) by Margaret Watmough (see link). On page 59, the author states: "The suffix -u may be added to a verbal (e.g. tur-, ces-), a preterite stem (e.g. alic-, zinac-) or a verbal abstract (e.g. zilaχnu). Intransitive verbs yield verbal nouns in -u of an active significance. Examples are zilu 'who presides' to zil- 'preside', zilaχnu 'who has occupied the presidency, president' to zilaχ 'presidency, magistracy' and lupu 'who dies, is dead, dead man' to lup- 'die'."
Quite a detailed and competent grammatical sketch, if you ask me, and it's similar to what I've concluded in my independent investigation. However, there's one tiny headache in this text I've quoted: Where is zilu actually attested? It turns out that zilu is a hapax (i.e. a word that has only been attested once in all known Etruscan inscriptions) which the author admits to on the same page. It's only found in ET Fe 1.2 and according to Studi Etruschi reads incompletely as: xxxpetlnaś arnθial xxx zilu xxx. Oopsy.
It seems to me that building a grammatical sketch on something so tentative is bad play and it perhaps reinforces my view that -u is rather a passive participle ending for only transitive verbs while -θ is applied to intransitive ones. My view then helps explain why *zilu is so hard to prove when zilaθ is abundantly attested, or why turu "given" is commonplace while *turaθ is thus far non-existent. These two participial suffixes show a complimentary distribution.