GB,
The glue up for the original top, which way does the grain run in relation to the leaf? That might limit your options if you want it to match. This may be a project suited for veneering, where you can highlight the fact that the leaf is different from the rest of the table.
I would also check with a local cabinet shop or a local sawmill. There are a couple near me who can plane over 15" wide. They might also have a wide drum sander if you do the glue up yourself and need a little help with the final finishing. For the 5 minutes it's going to take to set up and mill the wood, I doubt they'd charge you too much.
Another option I see on my local craigslist is people asking for the services of people with planers, jointers, etc for one special project. Who knows? some guy down the street may have a performax sander just sitting in his garage...
It's an interesting dilema trying to add a leaf after the table top is made, I can't wait to see what you come up with.
Good luck!