Ooh, I like the way you think, sir! I know mache is available around here since I've had it in salads at restaurants, but I've never seen it for sale in the groceries, so perhaps it's only obtainable from specialty purveyors.

Our neighbours had great luck with basil last year, but it doesn't appear to have self-seeded. It'll be a good filler if we can't come up with something more exotic.