Aside from ASTI, Arkance Systems seems to be the only other listed provider on ADSK's help page.
https://help.autodesk.com/view/FABRICATION/ENU/?guid=GUID-61CD687E-26A5-40BE-A2E6-E22DD0F6EA7C I would strongly suggest against the custom NC route.
While the feature itself did have bright potential, I do not think it has been maintained/upgraded in quite some time. While I have gone this route in the past for various machinery setups, I always found the feature to be clunky, glitchy, and a cause of countless crashes.
There are also downstream effects of a Custom NC setup as well, such as graphical views while viewing NC.... (Cut Color indication not clear/incorrect.... 300 circles of various sizes all throughout a sheet).... but when pulled up at the hardware, no visual issues, and cuts as expected!
Since you mention that it is just a header that is needed to be filled/added to your cut file, if you are a powershell wizard you might could easily achieve this.
While at ASTI several moons ago we created a "folder monitor" program.... this was an exe that resides in the system tray/startup that monitored the CNC folder where your cut files go.
Once a cut file with a specific extension (in this case your Mitsubishi's default extension) hits the folder, the powershell script opened the file, inserted a header/various data within the cut file, and then saved it......
As long as that's the only variable you're missing I don't see why you couldn't achieve this.
At the end of the day... If a listed organization is unable to do the work, and the software does not give users an easy, non error filled way to achieve it on their own, then the CAM user base is strongly at risk as new machinery continues to be brought to market.