Pickled Mustard seeds themselves are being seen more and more as a condiment (Tom Colicchio of Wichcraft offers on many sandwiches)
Recipe:
http://chubbyvegetarian.blogspot.com/2012/07/amik-pickled-mustard-seeds.html
That's how I read the question as well. But a pickle is a pickle. The seeds you use will alter the flavor but there really any hard core rules as to herbs and spices. It's what tastes good to you.
Yes, you can. You will have a slightly different flavor than if you used the celery (Rebecca above has good ideas for alternatives closer to the celery seed flavor) but you'll be fine and your pickles will be tasty.
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Recipe:
http://chubbyvegetarian.blogspot.com/2012/07/amik-pickled-mustard-seeds.html