In the meantime I found a way to extract the initrd from the TILO image. But trying an installation with it and the CDROM kernel (which I assume is identical to
the kernel inside of the TILO image) showed some issues (missing Debian ports keys, defaults to buster, debootstrap fails to install initramfs-tools, etc.) that
prevented a successful installation.
Please search the debian-sparc mailing list regarding the netboot topic,
we have discussed netboot in the past. Maybe this discussion can shed
some light onto your questions:
https://lists.debian.org/debian-sparc/2017/04/msg00002.html