As promised here are some A900 findings which I've accumulated so far.
1. Optimal ISO range is 320-800, 320 is the best, top limit depends more on your converter and taste, so decide for yourself. This means that in this range you don't underexpose and bump ISO if needed, but you don't go above the top limit and start underexposing. If you need high quality low ISO go with good ND or polarizer.
2. Wouldn't bother with ETTR - this camera is better than that, just shoot as you did on film, by meter, and your colors will be better.
3. UniWB is highly recommended, especially if you shoot scenes with dominating red or have weird light. Daylight is not safe either - more depends on actual colors in picture, so UniWB is a good safety guide.
4. Use Zone setting to adjust light meter bias - it goes in 0.5EV steps and -1 opens shadows at cost of highlights, 1 or 2 gives more space for highlights at cost of shadows.
5. You'll have hard time finding lenses which can outresolve this sensor. Not only because it is 24M, but also because it has weak anti-aliasing filter. In my experience Sony 50/1.4 is very good at this, Zeiss 135/1.8 is simply outstanding. I also like manual Zeiss Flektogon 35/2.4 through chipped M42 adapter, even though I'm not sure if it can outresolve the sensor. There should be others of course. This also raises requirements for tripod/head if you need really sharp shots - even with 2 sec. delay and mirror lock up, remote release and tripod I still get slightly different level of sharpness in two consecutive shots sometimes.
6. For M42 use chipped adapters programmed for each worthy lens exclusively. James Lao is recommended (http://eadpt.cn/eadpen.htm). I'm not affiliated with him and had problem with his product, but in my opinion he is still the best in this regard. You'll have working metering and proper EXIF info, both are important.
7. NR for long exposures - so far I don't find it useful at all, if your converter can suppress hot pixels better use converter, but jury is still out here. Need to do more testing.
Shameless plug goes here:
If you are on Mac, you may find that my converter (RPP, link in the signature) can "magically" make your A900 produce even more details, better noise and colors and higher DR. Give it a try