Along your lines of using bottle dregs. I have had some good success and especially with Jolly Pumpkin dregs.
I pitched one bottle into one gal of medium gravity wort. It was fermeting quickly with their sacc strain they use (WLP530) and had a nice pellicle after a couple months. I recently tasted this batch and it was over the top sour. Which I am a fan of but will probably blend to make it a more drinkable beer.
More info with pictures here:
So I don't really know how many bottles you need for 5 gallons but Old Sock's advice is good that you would be safe with a couple. Especially if you are planning to pitch the Roeselare pack.
I also suggest pitching more sacc yeast than is in the Roeselare blend because my Oud Bruin took way too long to show activity with only the Roeselare. And a beer that is goign to age this long is not worth risking the chance that other worse tasting bugs take over.
For stopping the bugs here is a good threaad. Actually Old Sock (Mike T) comments about his results.