This RnD would extend GeoServer to be able to return features with a z-coordinate, based on an attribute of the data (instead of true 3d which would store the 3d coordinates). This could be configured server side, or perhaps even better allow clients to pass in the name of the attribute that they would like to see as the z-coordinate. Ideally they could even do filter function transforms on it.
There's been some initial success, with hacky hardcoded values against a particular dataset. But it worked as a proof of concept and made some nice pictures.