We have recently added the possibility to easily create custom file readers in Python and I wanted to give it a try to see how well it works. The result: it works very well and now there is a SliceOmatic tag file reader available in Slicer (in Sandbox extension).
If you install Sandbox extension (in Examples category in the Extensions manager) then you can load the .tag file directly as a segmentation by simply drag-and-dropping the file to the application window.
I only tested it on a single slice and single segment, but it should work on 3D data with multiple segments as well.