The objective is to penetrate and steal the coins and then give me a PoC so that the issue/s are fixed.. rather than me depositing coins in escrow.
Have you also thought about the hacker breaking in, stealing the coins and going quiet without showing you how he did it? 
Think about it, if I invite attackers, they break in and find no coins, do you think they will report me how they did it? It will serve me no purpose. If you do not have faith, it is fine, move on.
Also, without any form of guarantee that they will find coins in there once they break in, there is going to be close to zero motivation that someone will even attempt to hack your site. @examplens' idea is actually good. Leave very little funds in the addresses and then send most of it to an escrow. Whoever finds the funds in some addresses, will receive a percentage of the funds from the escrow service. This will also ensure that the hacker will meet his end of the bargain by reporting to you how he cracked into the service.People should also be able to track and know if someone has already cracked everything or not so that they may not waste time hacking into a service who accounts have already been drained thus receiving no reward,