I do have a functional workaround, but it’s ugly.
I did find that it was possible to:
Drag the file into the Adobe Premiere timeline.
Choose File > Export > Media…
Set Format to MPEG4.
Uncheck the Export Video box.
Click on the Captions tab.
Set
Export Options to Create Sidecar File
File Format to Scenarist Closed Caption file (.scc)
Click Export
Drag the SCC file you created to your timeline. Adjust the captions so they line up with the audio track/existing captions.
In the Project tab, select the SCC file.
Right-click on the SCC file, and select Modify > Captions from the menu
Change Standard: from CEA-608 to CEA-708. Leave Stream as Service 1.
Click OK.
Choose File > Export > Media…
Set
Export Options to MXF OP1a
File Format to XDCAM HD 50 NTSC 720p60
Click on the Captions tab and Select
Export Options to Embed in Output File
Click Export
This will give you an MXF file with both CEA-708 captions for normal playback, and CEA-608 captions for outlets that downconvert to SD formats. But it’s sure not pretty or user friendly.