Just ordered five 5 inch wheel casters. They use Delrin bearings. I
haven't researched it, but I suspect that will help reduce grind
caused by dirt. Otherwise they might need to be expensive dust-proof
type ball bearing casters.
I might limit the turning radius of each caster, somehow.
They are stem casters. Mounting should be easier than a plate caster.
It will require drilling a hole for the stem, and then drilling a
large hole for the bearing housing to fit flush against the wood.
That might be doable using a spade bit. The casters need to be angled
toward the front of the skateboard. Mounting the casters/wheels is
going to be a challenge.
Also interesting is how exactly casters will cope with rough street
terrain. I don't think they can cause the skateboard to jerk left or
right, because that's just not how castor wheels work. If there is
forward momentum, the caster probably will follow the force direction
even if it's encountering an sideward obstacle.
The Ripstik has it easy since it's used only on smooth surfaces (as
far as I know).