You can get an HDMI extension cable (short - but long enough to reach where you want it installed).
As for mounting it? Hot glue?
I did exactly that. I got an extension cable (HDMI male to HDMI female), just long enough to reach from the Vampire to the empty space on the back of my A500+ (next to the Mouse port, under the floppy disk drive). You can dremel out a hole (rough shape) and then use a flat file to sand it down until you make it the same size as the female end of the HDMI extension cable. There is enough room next to the Mouse port or under the floppy disk drive, so location is a matter of taste. I chose next to the mouse port so that all cables went out the back.
Once the cable was held in place by friction alone, I put a drop of crazy glue to keep it in place and then hot glued all around it from the inside of the case (you have to remove the floppy drive for this, so be careful that both power and data cables on the floppy drive are connected back with the right orientation) It is very sturdy with all that hot glue, so I do not fear it will yank out if I pull on the external HDMI cable too hard.