Particularly for male singers, is there a relationship between where the singer's "comfort zone" (tessitura) and where the passaggios lie? For instance, will the upper range of a singer's tessitura fall in the zona di passaggio between the first and second passaggios? Is it possible for the tessitura to extend over the upper end of the passaggio region?
Yes, there are correlations. Passagios tend to fall on certain notes for certain vocal types, and tessitura is a function of vocal type. And yes, tessitura often extends a little bit above the second passagio.
According to the page below, the following notes roughly correspond to these classical vocal types... but there is individual variation.
http://www.singwise.com/cgi-bin/main.pl?section=articles&doc=DeterminingVocalFach&page=2
Passaggi Locations For Male Voices
G3 and C4 -- low bass
Ab3 and Db4 -- lyric bass
A3 and D4 -- bass-baritone
Bb3 and Eb4 -- dramatic baritone
B3 and E4 -- lyric baritone
C4 and F4 -- baritone-tenor or the robust tenor
C#4 and F#4 -- dramatic tenor
D4 and G4 -- spinto tenor, may also reflect those of a lyric tenor
D#4 and G#4 -- light tenor
E4 (or F4) and A4 (or A#4) -- high tenor
Passaggi Locations For Female Voices
D4 (or Eb4) and D5 (or Eb5) -- contralto
Eb4 (or E4) and Eb5 (or E5) -- dramatic mezzo-soprano
E4 (or F4) and E5 (or F5) -- lyric mezzo-soprano
F4 (maybe lower for a long middle range/register) and F5 (or F#5) -- dramatic soprano
F#4 (or G4), (maybe lower for a long middle range/register) and F#5 (or G5) -- lyric soprano
