Fusion of neuron with a S phase cell
This specific experiment has not been done. Fusing cells is difficult and it also leads to polyploidy. Cells are immortalized by overexpressing oncogenes (or viral replication genes); for example HEK293 cell line was established by transforming embryonic kidney cells with adenovirus. 

Now transforming a neuron would immortalize it but would also make it lose its function; for a cell like neuron, replication would compromise its activity for e.g. loss of established synapses. As mentioned in the comments by har-wradim, it was shown that transforming neurons with oncogenes leads to their de-differentiation. Another experiment of transforming neurons gave a contrasting result. Feddersen et al transformed purkinje cells in mice by expressing SV40 viral t-antigen from a purkinje-cell specific promoter and surprisingly it lead to degeneration of cerebellar tissue. Also have a look at this article.