Druid, I don't know whether this works or not as I personally haven't tried it yet. I recently picked up this tip from someone in the UK and they swear by it as you retain all the mussel's natural juices.

When you get fresh mussels let them soak in water for a while, letting them take some into the shell, tap water is okay. Now freeze them for as long as it takes for the meat to go solid, overnight is ideal. The next day tip a little boiling water over each of the mussels, but you must do this one at a time. The boiling water will make the shell open just enough for you to slide a knife inside and cut the hard muscle that holds the shell locked tight.

Once you have performed this little operation the shell can be opened wide, the other side of the muscle can then be severed, then you pop the still frozen mussel body back into the freezer.

You must do this one at a time because you do not want the boiling water to touch the meat to either thaw, or worse still, cook it. This way you retain all the natural juices and the flesh will not go soft on you and spoil. As you will be aware it is far easier to bait a hook with a frozen mussel, this way you can get better presentation and it won't block the gape of the hook.

I hope this little tip helps.