You should be able to replace all the mounts one at a time so you dont have to take the engine out or remove the axles from the trans. All youd have to do is make sure you are supporting the engine somehow, be it from under or top, and you can take them out one at a time. But once youve gone that far taking the extra 2 hrs to move the engine in and out may save some serious headache time.
But back to your original question; buy some sort of silver pen or white out so you can mark the belt and the pulleys. That way even after you have clamped it you will be able to see right away if you have moved it. The truth is that once you tighten a t-belt you really arnt supposed to loosen it and put it back on. When you take the tension off the system youre supposed to replace the belt. I guess if you had just a few thousand miles you nay be ok, but if youre up in the 30K+ you may want to just invest the $16-30 in a new belt just to be 100% sure.