Hi Bryan,
yes, that's of course a possibility, but I doubt that many users would like to activate hibernation mode just for interrupting a chess game (as it saves a huge hibernation file to the harddisk).

In the meantime I've found an other working method (at least in Winboard), but it's a bit complicated. The decisive point is, that the chessplugin has its own board representation and doesn't know anything about a position (or state) reloaded in Winboard or the engine. So in Winboard you have to save the whole game (NOT the current position!) and later reload this saved game again to let the chessplugin know the complete game (or move sequence).
When this saved game is sent to the engine (by the plugin), of course the engine should only execute these move, but it must not respond with any countermoves, and thus the engine has to be set into 'Monitor / 2-Players' mode before, so this method only works with engines which have such a 'Monitor' mode (most of them actually have this feature).

Here's a detailed description of how to abort a game and replay it anytime later:
For example let's assume you're just playing a game (with White) vs. Fidelity EAG V10 in WinBoard, it's your turn and you want to interrupt the game, then make the following steps:
1) set the engine into 'Monitor' mode (for the EAG V10 this is OPTION + c1 (twice!) + CL)
2) save the current state in the EAG V10 engine with "File > Save State'
3) save the game in WinBoard with 'File > Save Game'
Now you can exit WinBoard, do anything else on your computer or switch it off.
To resume this game anytime later, start Winboard and the engine EAG V10 again, and then make execute the following steps:
4) first set the engine into 'Monitor' mode again (OPTION + c1 (twice!) + CL)
5) now reload the save game in WinBoard with 'File > Load Game' (this sends all played moves to the engine via the plugin) and wait until all moves have been executed in the engine, too
6) reload the saved state in the Engine with 'File > Load State' (this restores all game parameters like level, times etc.)
7) before you can continue the game with your next (white) move, you have to execute 'Mode > Edit Game' in Winboard
8) now enter your move in WinBoard - this is sent to the engine (which is still in 'Monitor' mode, so it doesn't yet respond automatically)
9) switch OFF 'Monitor' mode again in the engine (OPTION + c1 (twice!) + CL)
10) now execute 'Mode > Machine Black' in WinBoard - I would have expected that now the engine should start/respond automatically, but it seems WinBoard doesn't send the necessary 'go' to the engine at this stage, so you have to make one final step:
11) execute 'Engine > Move Now' in WinBoard - now the engine starts thinking and you're in the normal game mode again, i.e. after the engine has moved, you can continue your game as usual.

The points 1), 2) and 6) could be omitted, if you don't care about the state or settings of the engine (e.g. already elapsed time, level, etc.), or if you re-enter the level manually before the last step 'Engine > Move Now', i.e. it could even be done without any 'Save/Load State' in the engine -
the only important things are:
a) in WinBoard save and load the whole game, NOT the position, and
b) switch the engine into 'Monitor' mode before loading the saved game!

Unfortunately for Arena this method is not working, because Arena doesn't send the moves of a reloaded/replayed game to the engine (at least not Arena version 1.1 included in CB-Emu), and I've not yet found a workaround for it in Arena.

Regards,
Franz