unpack in /tmp.

tmp # zdb -e -h -p /tmp/ chp

History:
2012-03-07.18:32:14 zpool create chp /tmp/zfsdisk
2012-03-07.18:32:14 [internal pool create txg:5] pool spa 28; zfs spa 28; zpl
5; uts sigmaii 3.1.10 #1 SMP Sat Jan 21 22:07:15 CET 2012 x86_64
2012-03-07.18:33:07 [internal snapshot txg:16] dataset = 34
2012-03-07.18:33:12 zfs snapshot chp@withkey
2012-03-07.18:35:01 [internal destroy txg:39] dataset = 34
2012-03-07.18:35:06 zfs destroy chp@withkey
2012-03-07.18:35:08 zpool export chp
sigmaii tmp # zpool import -T 38 -d /tmp chp       
Pool chp returned to its state as of mer 07 mar 2012 18:33:22 CET.
sigmaii tmp # cat /chp/.zfs/snapshot/withkey/chiave/*
12
56
90




As an alternative: open zfsdisk.challenge with a hex editor and invalidate
(change at least one byte, its checksummed!) every uberblock after the
provided timestamp. There are 4 copies of them. 
Tried, in 30-40 minutes it can be done.