Huh. Didn't even think of using ExecMethod. New script is:

Dim $sUser, $sDomain
$sFileName = "SNR Dodgy Constructions Pty Ltd.MYO"

$oWmi = GetObject("winmgmts://./root/cimv2")
$iIndex = 3
$oEnum = $oWmi.ExecQuery("SELECT * From Win32_Process WHERE CommandLine LIKE '%%$sFileName%%'")

For Each $oInst in $oEnum
	$oParams = $oWMI.ExecMethod($oInst.path_, "getOwner")
	? "User: " $oParams.user

Did exactly what I wanted it to do. Thanks, greatly appreciated. Now to work out how to log off the session for the users it finds...