Is there any way to recover, to compile a custom kernel when the generic kernel
fails to boot successfully but the installation kernel boots, and allows
installation of NetBSD?  Is there any way to mount the hard disk partitions and
run the compiler on syssrc, using shell sh, having booted the installation
kernel?  Or are too many things missing?

Only other way I can think of is to cross-compile from a Linux system, but I
remember being told that was impossible or next to impossible.  Any way to
telnet to a remote NetBSD system and compile a kernel there?  That does not seem
likely to me.