We actually have something planned like this for in the future. Unfortunately, none of us are really voice actors, and we don't have a studio on-hand at the moment, so we'd probably have to take a look around some VA sites.
Well that is fantastic to see you already have the idea planned. I would do this towards the end of your post production and substitute (if technically needed) during an earlier development period with just standard voice fill-ins. A perfect example is Vale’s Portal 2 and what they did with the character Wheatley as they developed their game. So if there is any last minute changes to a fundamental game mechanic you don’t have to ask a VA to come back to reprise their role. When you get to the end you can actually advertise your looking for VA’s and then have them act out their individual parts.
There are plenty of talented VA’s out there that are not officially in the business yet. In fact some of these guys would jump at the opportunity to make a better portfolio for themselves. As a matter of fact Black Mesa still has some of their VA’s they got originally from a few years back. All they did was advertise on Moddb and their website to have people send in their unedited voice reels, of specific dialogue lines from original Half Life. Of course they did have a independent sound designer I think on their team for their selective process.
It would be amazing if you guy’s could get someone who could perfectly imitate Udo Kier voice for your game. Or better yet get the man himself, but that would be insane to pull off.