Well, at least at my airline, ZED tickets have to be issued for a particular airline, for example LHR-JFK for British Airways.
But as ZEd tickets are priced based on distance and taxes, any carrier that your airline has an agreement (as an example: continental, american...) will accept the tickets for the same route. Iberia has a good costum when we issue ZED tickets which is to write at the endorsment box the following sentence: TICKET VALID FOR ALL ZED AND ONEWORLD IB CARRIERS. This sentence should advice the check-in agent that even if the ticket is not issued for his airline, she could check the staff agreements and accept it.
Most airlines will accept it without problem. So you dont have to buy as much tickets as carriers fly the route.
Other thing is to accept the ticket for a different roure that is printed on the ticket... that is the big problem for most carriers, at least at foreign stations where they are not very used with ZEDs.
There are some airlines that it does not accept ZEDs without exact city pairs as a rule. It has happened to me with Cathay Pacific and others. They did not accept a ticket issued HKG-SFO to fly HKG-LAX, even the ticket cost the same.
Others are much more flexible, specially the European and North American.

Nowadays the industry tendence is to change all ZED paper ticket to Electronic tickets, and even there is a web site that allow some airlines to issue tickets for other airline via web, with a employee code (www.myidtravel.com) You can be at home and issue your own electronic ZED tkts with several carriers. So I believe this will be the future for interline agreements. So it will be the end of preparing a list of ZEDs and going to the airport, and wait for issuing, and then listing, and all these things. And when comming back going to the airport to refund all unused parts...

Iberia has now started the web issuing project with Finnair, SAS Scandinavia, AirBerlin and Spanair. I have not tried jet.
I have tried electronic tickets but issued at the airport, so they print out a copy of the itinerary and the electronic ticket number.

I hope it helps.

John.