I think one way to do this would be to simply modify your grub.conf config file in the post section and append it.
For instance, you could use sed to match the kernel line and add your new parameters at the end.
We don't have an explicit mechanism at present.
Does this work for you?
Edit: A really simplified example of how you could do it, you'd probably want to tweak it fairly significantly
sed -i -E 's/^\tkernel(.*)$/kernel\1 my.flag=1/' /etc/grub.conf
I should be able to do what I need to do using the post section - I must've skipped over that part of the documentation when I was searching for the answer.