"Kazzagin, my wise friend, have you ever wanted a squire? For the chance to serve one as wise as yourself I would even give up most of my thieving ways and I could be useful to you, I am very good at finding and disarming traps"
(Bluff check, for some of that anyway)
Mardred, safe in the knowledge that he's not in a Union or Guild, and so therefore exempt from the lottery in the Great Hall, falls to one knee before Kazzagin and pledges his service.
Mardred can barely disguise his deceit throughout his stumbling, awkward offer of loyalty/penance to Kazzagin.
There's no way... absolutely no way that Mardred can be picked for the expedition, no way- none what-so-ever.