You are correct, simply assign a different CSS to the VM ports (if using SCCP integration) or SIP trunk (if using SIP), make sure this CSS references only the PT that routes all day.
Keep in mind that if calls arrive from voicemail via AA for example this change will remove the time of day routing, so if that is needed you may want to look at other workarounds, for example you can assign alternate MWI on the mailbox and then create a translation pattern for example to translate the alternate DN into the "open" partition.
HTH,
Chris