Hi,
I've never put a trailer hitch on a TR4, but I know the type of bike rack you are talking about and imagine it would be possible to weld up a receiver frame that bolts to the body/frame mounts on either side at the rear, and maybe the bolt in the center of the spare tire well that goes all the way thru the frame tube. Definitely a custom job. The biggest concern would probably be ground clearance.
My solution for oversize items was just put the top down and put things in the "back seat", such as it is. But, looks like you have a surrey backlight, ruling that out.
Another type of bike rack might be possible to clamp to the sides of the trunk, much like a "ski rack".
Or, it might be possible to rig a roof rack that fits in the holes for the surrey top.
Good luck! Let us know what you end up doing.
Cheers!
Alan Myers
San Jose, Calif.
'62 TR4 CT17602L