I agree with Steve that TG 20:08 states that toeboards may be dispensed with on stairways, however it also states that toeboards must be fitted to all platforms where there is a risk of materials falling.
Therefore the dilemma you're left with is, say the brickie is on his way up the Haki with his bucket of tools and stops at a landing for a rest, then accidentally knocks his bucket off, you could be at fault for not providing toeboards.
A risk assessment will always show that toeboards will be needed unless no person carrying any tools or materials at all is liable to use the staircase.