With kind regards,
Where do you need it? One way can be by adding a Custom Property to the project. From there you can reach the control tree and add a HTML control for the LINK and SCRIPT elements you need. It will only work inside the Edit form and you won't reach the page tree for example.
Another approach is to hack the Masterpage below Program Files or if you copy that whole chunk into your repo. If you're running 6 now it's probably gonna stay on the same 6 version so you're not likely to do much extra work keeping your hack in place.
I need it to load into the client's browser when requesting a page. Hopefully we can load it only for users who are actively in the Edit mode, since that is my primary focus. Otherwise I will build my own solution for limiting the script to Edit mode.
I will not have simple access to the project's code, and therefore hope there's either some kind of master template that can be modified, or like in the case of EPi9 there is a config file that I can modify to load the resources.