all that they have to store is "Address so-and-so owns so many shares, until we get a BTC signed message to tell us otherwise.
All that requires is a text editor that has a search function (so you can zoom quickly to the lines about a particular user or bitcoin address) and an email address where people can send you their instructions.
If you start adding in automated checking of the emails for certain syntax of commands congratulations you are on your way toward emulating MPEX.
-MarkM-