Why do we still have a single FDR and a single CVR? It seems reasonable to have a master recorder that collects and collates all the relevant data streams, but I don't find it so obvious that there should only be a single copy of the data.
With current technology, the weight penalty of having a number of slave recorders, driven from the master FDR and CVR, distributed around the airframe should not be prohibitative.
There must be quite a few locations where a hardened module would have a good chance of surviving fragmentation of the airframe, or could be released when subjected to significant water pressure. Winglets and the top of the fin seem to be reasonably plausible places.
In some locations, the slave recorder could be packaged with radio location features, as suggested by the OP. In others, it might be arranged to release a dye that would spread and help locate the stave (or even the primary recorder) after an accident.