Since they're both Linux, maybe you can have things set up so either OS can resume from images saved by either OS? If the hibernation image includes all of kernel memory, including the code, then it doesn't matter which kernel resumes from it. I'm not sure how that works.

Also, maybe if your laptop has ACPI hibernate support, Linux can hibernate that way. Then hopefully it always resumes when powered on, instead of booting to GRUB? Again, not sure how things work.

It might be possible for you to get your OSes set up to not use the swap partition if there was a resume image in it. This isn't going to help if they both mount any of the same non-swap partitions read-write. Maybe there's a mount option to refuse journal-replay (which would mess up the hibernated state).

If none of those things pan out, I think your solution is about the best you're going to get. I don't think grub knows how to look at a swap partition and figure out if it's in use. If it could, then maybe you could have some logic in your grub config to only allow booting the SAVEDEFAULT entry. (It's really unlikely that grub could figure out which OS had a hibernation image, without using SAVEDEFAULT.)

references:

http://www.linuxquestions.org/questions/linux-general-1/how-grub-knows-there-is-a-system-in-suspend-hibernate-state-742782/

https://superuser.com/a/405922/20798 appears to be just about how to get the resume=... always on the kernel commandline, not changing on the fly.