I’m not convinced that it’s a valid word, because the *-eity* suffix denotes “a noun of quality or condition corresponding to adjectives in *-eous* suffix” [OED]. *Liceous* for *licit* is at best doubtful.

However, in the absence of authoritative data such as the OED might provide, it would be reasonable to assign a pronunciation based on other *-eity* words. In these (at least in British English) the *-e-* is stressed, the preceding syllable is unstressed and possibly reduced, and the pronunciation might therefore be presumed to be /liˈseɨti/.

Pronouncing an unusual word in a reasonably standard way may help the hearer to spell it and thereby to understand it: /liˈseɨti/ is likely to be spelled *liceity;* the *lic-* part is probably to do with *law* and the *-eity* part denotes a “noun of condition” and thus the word may be presumed to mean *lawfulness*.

Footnote: Documents of the Roman Catholic Church are always published in Latin first and then translated, so it’s entirely possible that a standard Latin word for *licitness* — such as *liceitatem* — has been clumsily translated as *liceity*.