i'll one up you (given he wants to assign all to 1 agent)..
C:\BRK is the location of BESUserAdminClient.exe (and subsequently my input file). technically you could copy the .exe, .dlls and .config file anywhere to any machine. users.txt contains smtp addresses, 1 per line
Get-Content c:\BRK\users.txt | % {C:\BRK\BESUserAdminClient.exe <insert BAS or AD creds here> -change -assign_static_mailbox_agent 1 -u $_}