Now that UniMart is no longer part of BioMart, is there a way of using Bioconductor annotation packages to convert UniProt IDs (e.g. P15172) into protein names e.g. (MYOD1_HUMAN). These names don't seem to be available with UniProt.ws.

They are available - they aren't protein names however. It's just the HUGO symbol and species designation, which UniProt calls the 'entry-name'.

> library(UniProt.ws)
> up <- UniProt.ws()
> select(up, "P15172", "ENTRY-NAME","UNIPROTKB")
Getting extra data for P15172 NA NA etc
'select()' returned 1:1 mapping between keys and columns
  UNIPROTKB  ENTRY-NAME
1    P15172 MYOD1_HUMAN