Just for completeness: further investigations told me that the µPD78312 is a member of the NEC 78K/III family, of which the µPD78322 is also a member.
That in turn led me to
http://kaele.com/~kashima/software/nec78k.html , which has a downloadable 78K/III disassmbler. Appearently it is only a demo version, allowing at the most a code size of 8 kbytes, but there's nothing stopping you from splitting the input file into 8k segments and disassembling them one at a time.
Seems to work under Linux using Wine too.