The University of Minho developed a patch that gives this functionality in JSPUI: https://wiki.duraspace.org/display/DSPACE/RequestCopy .
In implementing it for the XMLUI, I would like to alter the functionality so that the process looks something like:
a. User clicks on restricted item link
b. System responds with current behavior, adding a link/button to request privs to view item
c. Clicking link to request privs to view item sends e-mail to author of item: includes Yes or No options
d. Clicking yes sends e-mail to User with link to view item
e. Clicking on link creates 'ghost' group with read access to item, including associated bitstreams. (Probably time-limited; one month?)
This process would improve on the original process of sending the bitstreams to the User via e-mail; large documents often exceed outgoing mail limits, or incoming mailbox size restrictions. However, it also presents possible problems in trying to bypass the authorization system.
I'm just starting to trace paths into the guts of XMLUI, so the general process is still in flux. I'd welcome any input, particularly regarding how to temporarily grant viewing privs to the restricted bitstreams.