Yep valet sounds good.

Here's how it could work:

You need:

Valet d00ds (yeah it costs a bit...)
rent some bike racks (heck even some ladders rigged up right can work so long as the back wheel is parked in them)
super long thick cable locks (bike shops carry them, about $25 w/locks)
Valet tags (and possibly something to attatch them to bikes)

The locks are the only high buck cost (well unless you have to buy tons of racks)...but what it allows is tehy can lock every bike (even if the customer does not have a lock), and that also allows the valets all to have key access (no losing keys).

The advantage is that you have a re-usable solution, and you could possibly rent out the racks to any other group in the area that wishes to do the same.

Only problem I see with not having some kind of rack at the least is a fustercluck of bikes on the poles, leading to bikes being locked to other bikes. Valets might be able to do something about it, but depending on how many show it may need more.

I have no clue what the turnout would be, but I doubt any more than parking for 30 would be needed. If poles can do that, sweet!