With 1.8/1.9 our server has moved to a new map, but we'd like to keep the old map around.
I plan on linking them with a single portal by manually tweaking the portal config.

To keep things looking nice, it'd be good if there was an option to disable the creation of new chunks on certain worlds :)

You mean, just to keep new terrain from suddenly appearing at the edges of the old map? Just have an arbitrary cutoff?

There might already be plugins which do this... however, it would probably be trivial to add a 'static' flag to the world config.

I meant to keep the terrain generator from generating new chunks on an existing map.

The static flag sounds like the right idea :)

One complication; if a user portals into the world from the nether, and they're no longer within an existing chunk, it may cause problems :/