Contacting the Tukaani developer(s) is currently equivalent to contacting Lasse Collin. IRC is somewhat preferred over email, but both ways are fine.
The project IRC channel is #tukaani on Freenode. Lasse's nick is Larhzu. The channel is usually pretty quiet, so just ask your question and someone may wake up. You have a higher chance to find someone to talk to if you are on the channel 12:00Z/20:00Z (that is, from 12:00 to 20:00 UTC).
Lasse's email address is lasse dot collin at tukaani dot org.
Sometimes Lasse is a bit slow to answer, but if you don't get a reply within two weeks, assume that the email has got lost, and try either resending or use IRC.