I am struggling with exactly the same right now too. In my case, I am trying a second-hand D1 waterproof suit before making the final decision to buy it or not. Today I did the third dive with it.
The dump valve is located on my biceps too and indeed it seems practically impossible to dump air while in horizontal trim. The valve simply can't get to the highest point with horizontal body unless you turn upside down or something.
From the trials I've been doing so far I fount that the fastest way to dump air is to turn my whole body vertical. Then the dump valve woks great releasing a lot of gas quite fast, but of course that's far from ideal. The other way is to go off-trim i.e. head up a bit (say 45 degrees), wait a bit so that air gathers around shoulders and then by raising the elbow some air is released. My problem so far is that this way the air escapes the valve too slowly hence I need to stay in this "strange" position (body tilted up, with elbow stretched upwards, and feet feeling the squeeze) for quite some time to release enough gas.
All these with the valve fully open of course. I can't even imagine doing these with even partially closed valve.
Another thing that bothers me is that with BCD you learn to plan ahead (eg release air before becoming too buoyant). With the drysuit on the other hand, in order to be able to release air you should have already assented enough to have "expanded" air in the suit ready to escape through the valve.
I guess I just need more practice, but of course any guidance or extra info will be much appreciated.
Thanks
