Well, it's taken a while (mostly trying to understand how all the scripts fit together), but I finally have a complete mini-mission arc in the simple campaign. Nothing special, it's 2 missions linked by Intelligence Reports, and the locations have to be static due to the setup, but I think it makes a good basis for further arcs later on.
If anyone wants to have a go, just extract the ZIP file to your main folder (so that /mods sits alongside /base, /build, etc). You should then be able to either use ui_push mods in the console or simply run ufoinvasion.bat from the main folder (kudos to neonin here, because I totally copied this idea from his swish mod-pack).
Criticisms and ideas are welcome, so let me know!