The standard rifle is available for import and you can optionally buy a spare 12.7x108mm barrel/conversion kit. Of course, the 'standard' fifty is included with the basic rifle package. The optional caliber is not practical, but it would be interesting to have, if it can be occasionally fed, to some degree.
It is not the Form 6 approval/disapproval but more of the Bulgarian export approval ALONG with the cost of shipping that ammunition is very expensive. The airlines have some crazy hazmat prices and even restrict routes depending on origin and destination country.
Now getting Bertram to make you a batch of brass...that is an easy import and he already has an established importer that is approved by his Gov for export already.
I suspect your issue will be in reloading - primers. Possibly they could be reworked (machined and bushed) to .50 BMG primers. No idea how the primer pocket compares to the .50 BMG. Suggest calling CDVS and asking.
Actually, you could go to https://www.rccbrass.com/ - have them build you cases with a .50 BMG primer pocket. Expensive but, like many things, relative to your level of interest.