I made a bike carrier for my +4 so that I can carry a small tandem across the back of the car. The carrier attaches via a bracket made from two square section tubes bolted to the chassis (Williams carried out the work). The carrier itself is made from two smaller section square section tubes that slide into the chassis mounted tubes and this then gives you two rear facing bars that you can mount a standard bike rack on (effectively two roof bars). I'm sure this approach could be used for your bikes.
The other advantage of this approach is that you can still use the rack for luggage.
Below is link to the original post and photos.
http://www.talkmorgan.com/ubbthreads.php/topics/232817/Bike/Tandem_carrier_for_my_Plu#Post232817
