Sammydog,
Couple of things.
If you put any type of bike rack on the tow bar you will have to include the rack and bike weight in the calculations for the towball loading. For example, I have a very similar type of camper trailer. Fully laden with gear, with the spare tyre and a large toolbox on the drawbar, the ball weight is approx 65kg. Our car has a load limit of 75kg on the ball. So I can't put much more weight on the towball. You need to check the load limit for your towbar and just make sure you are not overloading it by adding the bike + rack to the towbar.
I have seen a number of trailers like these with bike racks fitted to the drawbar. And incidentally the trailer in your photo looks a lot like my trailer. Did you get the vertical spare tyre mount? If so I have seen bike racks attached to the vertical bar that face to the rear of the trailer. I have a large toolbox fitted to my drawbar, behind the vertical spare wheel mount, so a bike rack there is out for me. And to be honest the toolbox is worth its' weight in gold for the convenience factor alone.
As mentioned above go to your local trailer fabricator (yellow pages is your friend) and have a chat about your options. You may even be able to get something similar to the boat mounts that can be put on the top of a trailer.
I find myself in the same situation as I am unable to transport my bikes which is very frustrating. Personally I have been considering roof racks mainly because at this time I have little other option.
Hope my ramblings are of some use to you.
Cheers,
Michael.
CBG.