Hi Chris,
Thanks for supplying a bit more detail, I think I have a better grasp now of what you’re trying to achieve. I also think that we may be able to solve this in combination with a similar feature request, and that is to be able to enter project tags on invoice creation.
If project tags could be entered on invoice creation it would mean that we can then go one step further and pre-fill this box depending on the client selected. This would work providing you are creating your sales invoices from the general invoice editor, rather than via bank tagging or bulk imports.
Let me know if this would work?