Well, it's not very elegant, but you can try the following:
<?xml version="1.0"?>
<button display="icon" icon_size="large" label_pos="right">
<label>Copy to Batch Select</label>
<tip>Copy filenames of currently selected files to batch file for re-selection later...</tip>
<icon1>#clipcopy</icon1>
<function type="batch">
<instruction>echo @echo off>select.cmd</instruction>
<instruction>echo "C:\Program Files\GPSoftware\Directory Opus\dopusrt.exe" /cmd select none>>select.cmd</instruction>
<instruction>echo "C:\Program Files\GPSoftware\Directory Opus\dopusrt.exe" /cmd select {o}>>select.cmd</instruction>
</function>
</button>
Check out this thread if you're not sure what to do with the button code above...
Maybe be nice if GPsoft added some kind of argument to the Select command like FROMLISTFILE or something...