I imagine I could use turkey stock instead of water when cooking a box of couscous, but that boiling pasta in stock wouldn't work so well. In what kinds of preparations could I use stock instead of water? Could I use a 1-1 substitution? (I've got a ton of turkey stock in the freezer, and I'm looking for ways to use it.)
Any recipe where the water is part of a sauce or is expected to be absorbed (including rice, couscous, and yes, pasta too), you can use stock instead to increase the deliciousness. If you have an opportunity to add flavour, why waste it?
Of course, there are some caveats to consider when making the substitution:
As for the right ratio - that depends entirely on what went into the stock, how long it was simmered for and whether or not the stock itself was reduced prior to storing/freezing. You're definitely going to want the same total amount of liquid, but if the stock is highly gelatinous then you might want to dilute it, and conversely, if the stock is very weak then you might want to reduce it. There really is no golden ratio, but I'd probably stick with 1:1 for a "typical" stock.
Well, first off you've probably noticed that turkey stock really takes over in anything where you use it. It's not as flavor-neutral as chicken or beef stock. They add richness, but not necessarily a heavy chicken or beef flavor. Turkey stock, on the other hand, makes things taste strongly of turkey.
Apart from that I think you can use stock most places you'd use water or broth. Because a proper stock is usually a good deal richer with gelatin you'd probably want to thin it with water in many applications. For example, you'd probably get bad results using straight stock to boil pasta because the stock may be harder to absorb. But thinning it with a bunch of water would still get some extra flavor into the pasta, and reduce the possible cooking difficulties.
I'd consider stock to be just another flavorful liquid (thanks Alton Brown), to be usable in exchange for others like wine, etc. You need to be conscious of the gelatin aspect and mindful that some substitutions will be more successful than others, flavor-wise, but it's always worth considering if stock might be a good substitute for any other flavorful liquid.
We make rice with stock all the time (though if it's a really rich stock we cut it with as much as 50% water), and it's great for poaching chicken for a chicken salad.