Either the kernel can unconditionally invoke /kinit, which then would invoke the users /init if present, or the swsusp can be a separate initramfs binary which the user's initramfs gets to invoke (the second is arguably neater, but requires minor changes to the users initramfs.)
The Ubuntu initramfs doesn't use kinit, and it would be nice if we
weren't forced to. We do a number of things in our initramfs (like a
userspace bootsplace) which we need done before most of the things kinit
wants to do take place.
kinit is a nice default tool but longer term, I almost imagine it as a
busybox type of setup. Either you say "go" and it brings up the system,
or you call it with an argument, change argv[0] or something to get just
the functionality asked for.