How do these work if I don't have a hydration pocket in my tried-and-true old Gregory frame pack? It fits in top (main) compartment, but are there issues when it is full of H2O and the rest of that comparment is full of clothes/cooking gear?

Another solution would be to tie short looped sections of paracord to the top of each vertical frame rail, extending into the main pack area. Attach lightweight 'biners to these and to that you can easily clip the bladder. Or you can simply tie a section of paracord across the opening and clip the bladder to that. These solutions are not ideal, but I have used something similar in an older ALICE pack a while ago with good results. The main drawback is that you still don't have a dedicated sleeve for the bladder, but it is what it is, just work around it.

I put my hydration reservoir on top of the pack between the toplid and the main pack body. First I close off the main pack with the draw string and then position the reservoir on top while the top lid, cinched down, holds it all together. What this does is helps to keep the water flowing (the outlet is higher than in a dedicated hydration pocket therefore the water flows easier) and is easier to fill on the trail. I have never had a leaking Big Zip when closed properly. And with the new closer system, this is easier than ever. I think you'll like this set up.