Is it possible to connect many (20+) SPI sensors to an Arduino Mega? I am making a system for measuring whether or not there is vacuum present in big steel containers. They are located up to 100 meters from the control cabinet. 

The question is, how many SPI sensors can I successfully use at the same time with an Arduino Mega?

I have two main concerns, and there are probably more things I didn't think about.

Signal strength
The shared lines for CLK, MOSI and MISO will need to be split up at several junction boxes and led to the different sensors. Since the voltage out from the Arduino is fixed, I'm fearing instability because the signal is weaker for some sensors, has someone done something similar? What is the greatest acceptable voltage loss for these lines to an ordinary sensor? 

Processing
Will the SPI library be fine coping with this many sensors?

The refresh rate can be set to extremely low, thinking 0.3Hz. 

The alternative to all this is to go for 4-20mA sensors and have a 0-5V converter circuit for each sensor, near the controller.

Any inputs would be greatly appreciated!
You can control as many SPI devices as you can set the CS line for. You can even control all the CS lines from a SPI device. A SIPO register will help a lot there.

However you are probably better off using a proxy board to actually connect to the sensors and then connecting the mega to the proxy to communicate the measurements using a connection that is more robust over long distances.