If you're worrying about actually what can go wrong doing this (as opposed to breaking some "cardinal rule" about solo diving), why not just do everything you want to practise in 4-5 feet of water where you can stand up? What skills can't be practised that shallow (in fact, that's the best depth to practise buoyancy/hovering)?
I'm also jumping in late and skimmed some of the posts. Mustard Dave mentioned a good point -- the life guard may not know how to get you out of your gear assuming he can get to you in enough time. One nightmare scenario this suggests is jumping in the deep end without your air turned on. You're on the bottom so the life guard can't see you. You can't inflate your BC so you have to kick up. Most new divers are overweighted so you may not have enough energy to get to the surface. You might be able to ditch your weights and swim up. Another choice is to ditch your equipment and do a CESA. My advice to you is what TMHeimer said above.
P.S. My cavern instructor related a dive accident to an experienced cave diver. He entered 4 ft of water without his gas turned on or his BC inflated. As he stepped in he slipped, turned upside down, and was laying on his doubles facing the surface. Because of the weight he couldn't right himself and standup, and he couldn't reach behind him and get the gas turned on before drowning. He died with at least 180 ft3 of unavailable gas in perfectly good equipment, while staring imploringly at the surface barely an arms reach away. Unfortunately, he didn't have a buddy and no one witnessed his accident.