Mac repair USB Drive
There's a utility called DasBoot which allows precisely this:

http://subrosasoft.com/OSXSoftware/index.php?main_page=product_info&amp;cPath=200&amp;products_id=193

However, I'm not sure whether it is Lion compatible. 

If DasBoot doesn't work, there is another, sure fire way of achieving this.

Simply install Lion (or Snow Leopard) onto your USB drive. You should be able to do so with a 16GB, since the minimum disk space requirement for Lion is only 7GB, and it may even work with a 8GB. 

Instructions here: http://www.maciverse.com/install-os-x-snow-leopard-from-usb-flash-drive.html

Then you can simply install the things you need. AFAIK this is what the Geniuses in the MacStore do, and probably how I would do it myself.