Really? I didn't think there was CoS bits in the PPPoE header... That's new to me. I'll have to Google that...
There's no way to set that in the MX. You can't set CoS at all on the MX as near as I can figure (DSCP only). If it's really CoS they want you might need to add a VLAN tag on the WAN side since that's where those bits are.
Unless someone else here has another suggestion I'd call support and see if they have some way to set this for you.