My suggestion is to give players (testers) any valuable things for what they tell GM's location of important bugs. For example, if someone reported to GM about the bug, which helps to quickly (read: illegally) to develop his character, he receives, for example, combat armor or anything else, at the discretion of administrators. Otherwise, after the beta test, you (and administrators) will have a sufficient number of non-closed bugs that may negatively affect the gaming experience.
Someone here suggested that all these things that GM's can give players, for the bugs will disappear after the first wipe, but corrected bugs will not appear again, so that's WHY it still remains a good idea, imho.
For me as a big fan of the original Fallout / Fallout 2 - Fonline is already really big reward. But, again, as the Badger said, this is not so, for most players. So, that's why they must be somehow rewarded.
P.S. And yet, you should not forget that 2238 played not only by Americans and French, for most of whom (like me) the reward is a good game, but also Russian and Polish, which SOMETIMES have a bit different motivation. I particularly talking about the players which were playing server TLA, and about a week ago, switched to 2238. And most of these players certainly will not be reporting to the bug reports.