in fact, i'll be doing that for my yard toilet, since nobody uses it. ID said can do it, he'll overlay the hole with tiles. he said something like also sealing the area around the hole with silicone, so that anything (smell or sewerage) will not be able to leak through. he said can't totally block up the hole with concrete, cos in case downstairs neighbour got any sewage pipe problem, can still come up to check if necessary. also, when u sell the place, new owner may want to reinstate the toilet bowl, so best to leave it reversible.