Hi Bjørn, 

Blanket PO’s I thought were only for purchases that basically would be pre-approved. This is probably why you only see  that it is only available to Sales Orders.

You do have the option to add a PO number to the contracts which I thought would be populating the service orders created by the PM’s but this is not what the customer wants as you say they want different PO for contract invoices (meaning you want the ongoing contract invoices on PO #1 and the PM Service Orders on PO#2).  There is no distinction when to use this PO number from what I can tell but to turn off the PO on the orders is managed through the nodes setup.  

I believe this to a customization request as there is not a method to indicate the PO is for the service order only and suggest logging a services request in the customer portal to have the services team review possibilities.