> No, alas.  `unlimit' expanded the data size limit from 16MB to 64MB, big
> enough to start Lisp (but not big enough in the long run -- is this 64MB a
> kernel parameter somewhere?).  However, when started Lisp now exits

There's this "#define MAXDSIZ" in vmparam.h which enforces the 64MB limit.
You can try to increase it a bit and see what happens (I never tried it).

BTW. Note that a sun4c cannot handle virtual address in the
     range [512MB - (4GB-512MB)], so you wouldn't want to set
     MAXDSIZ to fall in this interval.