• "horizontal" slabs would be crafted by combining 3 wood blocks of the same type horizontally. they'd be slabs of a wood block that's facing sideways. (i.e. the upper slab has a bark top, the lower slab has a bark bottom. (maybe also inverted versions?)

• "barked" stairs are a bit different:
  they have bark on both steps, but the inner wood texture on the front of the steps.
  they are crafted by combining a regular wood stair with bark, which is dropped when stripping logs.

Discussed this with @UpcraftLP on Discord and these are some concept screenshots of what we ended up with.

Slabs:
Image 1
Image 2

Stairs:
We thought of the concept adding new variants of every stairs block, just covering all the top-faces with bark, but in some cases this ends up looking very weird.
This one looks fine, though.
So we decided that a variant where the "core" texture would consistently be replaced by the bark texture would look the best.

Full album including concepts from #5