I only use steel rivets and I know a block of wood would not be sufficient to support the head. I posted a video here on Pinside a few years ago showing how to use the hand held roll over tool.....1st post on this thread...
Anyway, in that video I just support the rivet head in one of the jaws on my vice... I actually ground a couple indentations in the top of my vice for the rivet head to sit into. I do most of my work with a rivet press, but it cannot get to everything....so If I can't use the jaw of the vice, I use a 3/8" bolt that I also ground an indentation into (Thread end). I clamp that in my vice sticking up maybe 1.5". I position the head of the rivet on the end of that bolt and then, just like in the video above, hammer away.
The trick to semi-tubular rivets, is picking the proper length of rivet for what you are assembly. The bottom of the rivet (tubular end) should protrude beyond what you are riveting by approximately just under 1/8". This protrusion is what is rolled over. When you use the proper length rivet, as you pound (or press) the rivet, it can really only go so far before the roll over tool encounters the solid center of the rivet. Go slow at first.....If the work isn't tight, just hammer it a little further.
Also always remember to use a washer when the rolled over portion of the rivet would impact plastic (Just look at the bottom side of pretty much any ramp flap)