I had a similar question, a while back, here is the answer I received.

In short, no, there is no good approach to this :)

If you do go down the path of changing the global $user object, then

you will want to use session_save_session to make your code look
something like this:

// Don't save the session if the next block of code fails:
session_save_session(FALSE);
// Save the old user account someplace.

$user = user_load(0);
// Some computation....

// Something like this:
$user = $the_old_user

// Its now safe to start saving the session again:
session_save_session(TRUE);

</code>

If your 'Computation' in the middle there fails, then the users
session will be saved with the $user object you loaded up, i.e.
they'll get logged out. Annoying at the moment, but if you ever change
it to assign other users to the $user object then you might end up
with people being logged in as other people if the 'computation' fails
for some reason.