Field report:

Called the Plat desk with a 074-issued EMD voucher worth 800 Euro (EC261 haha) to book a ticket. The nice Hibbing rep wasn't sure what to do with it, but after a call to the JV line she found out that the trick is to build the itinerary and then process the payment through the GTS desk (or Intl GTS desk for intl itineraries). They don't take customer calls so you'll have to read your voucher number to the rep who will then place you on a brief hold and call to get it processed.

Easy

I used to use these on KLM.com, but now that you can no longer get free EC upgrades on non-DL coded flights, it was worth it to call.