This idea has been around a long time, and was first quite popular in Touring caravan tiny bathroom layouts, although the washbasin itself was made of strong composite, lightweight and large enough to easily be reached in the down position over the toilet, but folded up flush with the wall via an hinge mechanism when not in use. It tipped the water out into a back channel and out to the disposal point, but could equily be adapted to use the water to flush a modern toilet. The downside was though that anything that slipped into the drainage channel, or even the back up of soap deposits made them difficult to clean without a major dismantaling job, hence they tend not to be fitted to the latest models of touring caravans today. However, I have seen some tiny corner handbasins that may suit the job, as they take no more than 30cm of wall space into the corner of a cloakroom, and work off one small central tap, and.without the problem of having to lean up against the outside of the toilet pan to use it, not the cleanest of areas even at the best of times, and not idea for someone wearing floatly baggy style clothing!