Are we talking about the addition of Action Safe and Title Safe Areas (see Google Images for examples) these might be useful one day, when Terragen gets the ability to load in background plates (Note on functionality: Please insure that the Aspect ratio and image size in Terragen automatically adjust to that of the background plate)?
Right now Terragen has no Shadow Catcher (That I know of) so even if you could load a live action plate (or still photograph for digital matte painting work) (which you cannot right now) there's no way without a lot of manual to match Terragens lighting such as light direction, intensity and shadow fall and direction to that of the background plate!
Maybe one day Terragen will get these tools such as "Set Image as Background" in the image shader a common feature in other 3d applications, normally used to assist modeling but with Terragen such a feature would be use to have a photograph as a plate that you wish to match 3D terrain to, for the intent of digital matte painting. if one day we get this stuff then we can start talking about the addition of Action Safe and Title Safe Areas.
Regards to you.
Cyber-Angel