Not difficult at all -- in fact, it almost happens by itself. The problem is not with installation, the problems are security and server loading.

Any reasonably skilled server administrator can install Sage. But setting up user accounts and preventing server abuse are more important:

    sage: N(pi,1e3)
     3.14159265358979323846264338327950288419716939937510582097494459230781640628620899862803482534211706798214808651328230664709384460955058223172535940812848111745028410270193852110555964462294895493038196442881097566593344612847564823378678316527120190914564856692346034861045432664821339360726024914127

    sage: N(pi,1e12)

    (smoke starts pouring out of the server ...)

Wolfram Alpha solves this problem by timing out any transaction after a few seconds. I don't know how difficult that would be to set up in the Sage server.

You can install it via NFS and add a few cute icons for the users. Really easy!