Had a similar type rack on an Heartland North Trail TT. It slide out as opposed to laying down. It came with a goofy piece of square tubing that you inserted into a slot. You would have to wrap a bungee cord around it to hold the bikes all together. Then they would probably flop around on the road.
Something you might try is some of these
http://www.afterschool.com/p/hollywood-racks-t970-fork-block-1-bicycle-fork-mount-937523?site=CA&utm_source=Google&utm_medium=cpc_as&utm_term=FPS-246072&utm_campaign=GoogleAW&CAWELAID=500003340000186025&utm_content=pla&adtype=pla&cagpspn=plaYou could bolt them on to the rack and bunge the rear tire down. I have used them on our bikes in various forms for the last couple years. Just have to store the front tire.