Minecraft: 1.12.2
Forge: 14.23.2.2613
Content Tweaker: 4.4.1

Between the wiki and the project page there are two styles these can be called. So for the sake of completeness I have covered both

Checking with both of these produces the same results. Pictured is the slab block with fullBlock set to false. true (the default) produces the same result
block.fullBlock = false;
block.setFullBlock(false);

Checking with both of these produces the same results. Pictured is the slab block with lightOpacity set to 0. 255 (the default) produces the same result
block.lightOpacity = 0;
block.setLightOpacity(0);

This picture may be harder to tell as it is a dark block but the center-ish area of the 4 "slabs" (fake) is darker

Edit: Log says nothing about either style of using the field being invalid. No error.

Edit 2: with direction from Jared I also tried block.translucent = true;

so to recap.
currently the block has
block.lightOpacity = 0;
block.fullBlock = false;
block.translucent = true;
No log error.
Result

var slab = VanillaFactory.createBlock("slab", blockmaterial:cake);
slab.fullBlock = false;
slab.lightOpacity = 0;
slab.translucent = true;
slab.register();

This works

Yeah totally wasn't clear. 4.5 fixes it and now it works!

you mean it works after a bug fix or it always worked? cos I tried that series of settings. its literally the last one mentioned :P