Hiding a difficult transmitter is not that hard. You need to define the intent of a HAMfest T Hunt; is it to get newbies into the sport or just competition for established Hunters? Then there is the safety factor in walking around a HAMfest with sniffer antenna that may poke eyes!
For a few years now, since Dayton dropped their T hunts, I've been trying to get the Friday T Hunt forum to adopt having a Hara sniffer Hunt starting at the conclusion of their session. Use the last 15min of the session to teach attendees how to sniff using a 2m HT and body-shielding/aluminum foil bag/slotted tin can/frequency offsetting/squelching.
What I'm promoting is a 2m sniffer Hunt using just a HT and body-shielding or aluminum bag/foil - but no sniffer antennas. This equals the competitive field and gives a newbie a fair chance in winning, plus this is a needed skill to learn and master! The no antenna restriction is for safety as the Hunters weave through the HAMcon booths/patrons.
Depending on interests conduct an on Hara grounds HAMcon sniffer hunt each day, limited to HT and no directional antennas.