Might not be a completely automated setup for your requirement , but then you can use the below suggestion:
You can use unmanage scheduling utility available on your primary poller (All Programs -> Solarwinds Orion -> Advanced Features - > Unmanage scheduling utility)
Add the servers that needs to be unmanaged using the above utility , this would create a .cmd file. If schedule is known you can add this onto task scheduler on primary poller or in your case you can just double click on the cmd file to initiate unmanage when you require the same.
