So here's my personal experience that may help others. The Lufthansa lounge does have 12 lockers where (with a key) you can store your luggage. They are big enough for a carry-on and, perhaps, a personal item to put in there too. On the day I was there, there was basically no demand for these lockers. We didn't try, but if you have a long layover and wanted to leave your bag there for the day and go out into the city, I don't think it would be a problem.
As far as being hassled for "over-weight" carry-on bag goes, I could see it might be a problem. At the Croatia Airlines gate, we were not hassled on the Star Gold priority line. As we boarded, I thought I heard the gate agent mutter to a colleague that there was no need to "check" the carry-on baggage of the priority pax, as they were allowed extra luggage allowance. I didn't see what happened after we boarded, but I wouldn't be surprised if regular pax were then hassled. I did see a lot of bags that looked heavier than 8 kg, though.
BTW, if you can live with the baggage limitation, Croatia provides better service than BA on the route. First, T2 is way nicer than T5 -- especially if you are Star Gold. Second, BA charges for food and drink on their flight, while Croatia gives both for free to coach pax.
As far as overall lounge quality goes, my previous assessment of the various strengths and weaknesses of the T2 lounges remains. Lufthansa probably has the best overall lounge (especially when you consider its convenient location) for food and drink and overall comfort. United's is nice for its showers, bar and general decor (there's plenty of food there, too, but somehow it's worse than Lufthansa's). Singapore's is slightly disappointing (smaller and less inviting than the competition, but perhaps worth a quick visit to get a Singapore Sling, a scone with clotted cream, and some Asian noodles). The Air Canada Lounge is the "sleeper" choice. The food/drink are no great shakes, but the lounge is large, not so crowded and has nice decor. There are a few hidden reclining chairs in the back that are probably the best place to take a nap in the Terminal.
Overall, I liked T2 a lot more than BA's T5. It's just a much more humane experience. T5 is basically crowded and nuts. If you have oneworld status, though, it's not horrible.