Off the top of my head, as I have never pondered this before...mount the tower to the countertop and then use some PVC pipe and weatherstrip to take care of the gap between the top of the chest freezer and bottom of the counter. A bit complicated because you have to deal with getting the weatherstrip a good seal and the beer lines room so you don't pinch them. I can think of a few ways to do this, perhaps the most elegant is to insert pvc pipe into the tap tower and have the bottom attach to the chest freezer with a flange. Then set it up so that you twist lock it into place with some screws/bolts going into the flange...some angled shims added onto the top of the flange around the twist lock holes would help build good pressure against a weatherstrip o-ring. Then, add some screws to the bottom of the counter so you can unlock the tower/pvc pipe assembly pull it up and lock it back in place to the bottom of the counter temporarily while you roll out the chest freezer and provide clearance for the beer lines.
Just remember to make your beer lines long enough to handle the freezer rolling out. You may have some issues getting the beer lines to go back into the chest freezer when you roll it back under the counter though, especially if you have no room for your arm to get into that gap. Perhaps you could use some of the hose line weights on the beer line that they use on the extendable sink sprayers to get the line to drop back under the sink easily.