I haven't had any difficulty adjusting mine hot. I typically warm it up by driving it for 20 minutes, do three cylinders, then start it up and let it run for a few minutes and do the other three, and check the ones I did. Just letting it idle for 20 minutes doesn't seem to bring the oil up to full temperature.
Adjusting solid lifter engines with the engine running is a very dangerous idea. If the adjuster slipped you could easily have valve to piston contact and a very badly mangled engine, as well as tool or hand damage. Hydraulic lifters don't require you to stick a feeler gauge in a moving piece of machinery.
I have not spilled a drop of oil idling my 261 without the valve cover on it. Just park it on a slightly downhill level surface so the block is level and the oil will run right down the drain holes. The rockers don't move fast enough to splash at idle.
If you are unsure if your clearance is right it is a safe bet to leave them a bit loose. They may be a bit noisy but that noise is not causing any wear.
My BMW's valves have to be adjusted cold because if you did it hot you would stir fry your hands. However, it has an overhead camshaft. The Chevy valves are much easier to adjust.
