That would be doable, though probably pricy with recent models. The 2006-2008 ones were pretty normal, with memory riser boards sticking off the motherboard, and connected in via edge connectors on the PCBs. Due to the weight, the case would need some support for them, but not much. The 2009-2010 models would need a better case, as the CPU and memory sit on a separate board perpendicular to the main board, with lots of tiny fragile pins used to connect them.

Could probably make some money back selling the stock Mac Pro case empty to people wanting to stuff normal components inside them.