Probably much quicker to put the bar in a CNC machine and hollow it out, than it is to go through the molding and casting process. Then polishing up the cast bar carefully so as not to remove details or leave marks.
A CNC machine can probably hollow out the original bar in seconds.
I doubt these reputable houses are accepting these bars without doing their normal checks.
VERY unlikely. I've done a LOT of machining. Gold is !SOFT!. I haven't worked directly with gold, but other metals with similar physical characteristics that I have worked with (tantalum, lead) are VERY difficult to machine, even with CNC.
And I doubt that the sort of folks likely to be doing this have access to $100K CNC devices.
Making a casting mold, OTOH, is very "low-tech", cheap, and easy. The "hobby supplies" market probably has all anyone needs to do the necessary.