I think your original idea is the better and more flexible way to do this. What you do is have "no inventory" item for your "package deal". Then set up a script that is performed each time that you select an a product for your invoice. That script checks to see if the selected item is a package and replaces it with two entries--with the specified individual product IDs, but with discount amounts entered in to fields that discount the items. This should keep your inventory correct as you are still listing the individual items, but streamlines data entry by allowing you to select two or more items as a group or "kit".
Thanks for the advice, Phil.
To be honest I'm still a bit of a beginner when it comes to scripts. I'll do a bit of research to see how to put that script together.