You need ROM level support on a Mac. The stock ROM does one thing, boots the System Software. It would have to accommodate the fact that there may be a hibernation system image on the drive and load it if its available.

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But surely we could just run a standard application to dump the RAM contents to a file on the hard disk, and then another application which can be run after bootup (or an extension which does the same thing during bootup) to restore the RAM contents and automagically make the former state appear.
My only concern is that it might crash the system should a VBL interrurpt or a pesky PDS card stick its foot in the middle of the memory re-writing process...