Radiant Can Be Used For That Purpose
if the scripting is coded in the game that you are targeting. The way entities and entity properties are handled in Radiant does not in anyway put a limitation on what you can do with the entites.
For instance, take Custents the Quake 1 package. I have used it for machinima purposes in the past with Radiant as it has cut scene entities and actor properties built in.
It all depends on what is in the .def file that comes with the game set up.
Since I despise JK2 as a game and on a conceptual level, I can't help you with anything in relation to that particular game. However, if JK2 has built in demo recording capabilaties, it may not matter. Uwe Girlich's Open Demo can be used with any of the QuakeArena engine based games as the basis of the virtual 'camera' work. Here is the link.