This is actually a really tricky one!
Splashproof implies a degree of water protection (I think that we use an IP designation - see http://www.industrial-computer-enclosure.c.../IP_Ratings.htm in the EU) but added to this you will need accessibilty and optical usability. I've used solutions for video (there is a 'rubber' tube with glass ends and a drysuit type zip available for example, which might work, but although I've HAD to use one once it instilled NO confidence in me whatsoever and I'll leave you to work out what the general concensus is on such gear!) and I think that there were relatively cheap plastic bag type housings (perhaps from Ewa Marine) which you might just squeeze such a system as you describe into, although if you look on their website its difficult to find anything currently available (a large camcorder version might be a compromise).
In all honesty, I'd say that you really need to look very carefully at whether you MUST use MF in this 'splashproof' environment. As you will see from the threads indicated, solutions are readily available for high end 35mm style dSLRs and unless you have an absolute, over-riding need to use MF, they are far more widely available - and are even rentable. I'm NOT gouing to comment about the quality differences between MF and 35mm dSLR other than to say that you will almost certainly find MF both difficult and expensive to house, whilst 35mm dSLRs have a huge variety of options. I appreciate that your requirements depend very much on what you are doing (amateur/pro work, client's requirements/own desires, etc.) but housing MF is not a straightforward nor a cheap option!
