Good to see a pre-Grouping project taking shape. There will be others far better qualified than I to comment on SE platforms, but here are two examples that seem to offer some encouragement.
Halstead, a SER station opened 1876 and later known as Knockholt. This is a wooden station. The platform appears to be masonry, however, as it looks to be edged with brick (quite common a common alternative to edging stones). The view is given as c.1880.
Another SER timber wooden station of the period, Bromley (1878) is said to have had platforms "edged in timber". Quite what that means, I'm unsure. Considering Bromley, is it possible to interpret the edging at Knockholt as timber planking, not brick?
Whatever the construction of Knockholt station platform, the effect is of a smooth, rendered, platform face beneath the edging.
The only other example I have so far spotted is at the LCDR's Canterbury, later known as Canterbury East, of 1860, in a picture dated to the 1890s. Here the platform is very striking, with no apparent edging/coping or overhang, it looks like nothing so much as a huge concrete slab, you can even see where it's chipped at the edge to the right.