actually, at one time, and maybe they still do, porsche made a 911 motor for use in aircraft with timing gears, no chain, no belt. so it can be done, and it MUST be reliable.

In some domestic V-8 set ups, the chain is eliminated with gearing. There are two main reasons for it- 1)it insures valve timing through eliminating any possibility of chain stretch and 2) eliminates the possibility of chain breakage because of forces generated by radical, high lift cams. As the engine rotates, the cam undergoes a series of "jolts" as the valves are actuated. If strong and frequent enough, these jolts can concievably damage a chain drive, hence the reliability for uses like aircraft.

Mercedes has used chains in their aluminum engines for many years. The chain on my old 1980 450 is about 6 feet long. The engines are durable and there is no problem with thermal expansion and chains. My 1980 has 150K on it. The chain was replaced at 100K. This engine is far from a performance engine.

On the other hand my 400E has 32v and 4 cams. It is also chain driven with an aluminum block and heads. It could be considered a performance engine. It's redline is about 6K.

My 18v V-6 AMG supercharged MB also is aluminum with a chain drive. It is definitely a performance engine.

So I would tend to say that chains can be used with aluminum blocks.

I'm not saying they can't, but how are the chains tensioned and what mechanisms are there to keep that tension constant?

There are chain driven valve trains that use spring loaded tensioners.

also the amount of tension on the chain is such that the effect of thermal expansion would be miniscule.

The old one uses springs, plastic sliders, and an oil pressure piston thing. I have never worked on the 3 and 4 valve engines so I don't know how they are arranged.

Don't forget about the need to oil the chain/gears. But this could be a good thing. Only the crank seal and timing chain cover gasket need to be replaced, when it starts leaking, instead of the 4 seals now.