With Oshyan's help, we've dug into TG a bit more to automatically correct some issues with scene files. Now, when the job is a single frame job that our system has decided to tile render, we're making sure the ray detail region and padding settings are such that TG will behave more like the user expects. There were problems with missing shadows and the like when a scene file is tile rendered but those values aren't set appropriately. This could also result in a blocky output image if not set correctly, since each tiles light setup would disregard the lighting in the adjacent tiles. As you can imagine, that produced some unexpected (and difficult to troubleshoot) results. Those should no longer be a problem, as our system will automatically correct those values in the main and any GI pre-pass scene files.
Additionally, we've had issues in the past with the tile stitcher that was native to the render farm management software we use internally. It developed a painfully bad habit of reporting it was done stitching the tiles together when it hadn't actually completed...or would ever complete. We switched to ignoring what that software thought it was doing to using ImageMagick, controlled by our software. There were some ridiculous defaults in ImageMagick as regards EXR handling that had to be found and overcome, but that has been done. The result is reliable and accurate tile stitching for Terragen still image jobs (and the other apps that support tile rendering) no matter what output format you've defined in your scene file.
Phew! Lots of little issues circumvented. We're hoping this makes TG users happier.