My Yam has a bracket arrangement which sounds very similar to Bilge Rat's. I know that the previous owner, Alan, has trailed the boat hundreds of miles on this bracket without any problem, and looking at it's construction I'm completely happy. As stated, the PTT ram is always there in case of failure. I'm reluctant to jam in bits of wood in case this lifts the engine up and off the support.
Now, i used to have a rig a bit like Phil Davies, albeit smaller, but had the same conumdrum - no PTT ram, no substantial support bracket and not enough clearance to set it in the down position. So I constructed a sort of support shelf which extended from the back of the trailer. This took the weight of the o/b and provided a good location for the lighting board. I've posted a picture of it before, so I'll see if I can find it...
These two threads will give a bit more detail...
http://rib.net/forum/showthread.php?s=&threadid=1842
and
http://rib.net/forum/showthread.php?s=&threadid=1591