Some of you may find this useful, I ran into this problem where I needed to set the value of an extension attribute on a number of hosts from a list. The following script should take care of that. At the end, it will let you know that the host is finished, it will output an error if one occurs.

import-module activedirectory
$machines = Get-Content "C:\PATH TO LIST\feedme.txt"
foreach($machinename in $machines)
{
    Set-ADComputer $machinename -add @{extensionattribute10 = "VALUE IN THE EXTENSION ATTRIBUTE"}
    write-host $machinename " is done."
}