I have been looking for a block that would count entities in a given area and emit a redstone signal proportional to the amount of entities, and have been unable to find such a block in all the mods I'm using.

I feel like such a block would fit RFTools' theme and would be great alongside the Spawner.

You would be able to chose the following:

• Type: Any Entity, Specific Entity: , Entity Group: <hostile|passive etc...>
• Area: Insert a Card that would determine the area the Entity Counter
• Redstone Signal: the amount of entities per increment of redstone. For example X=4, if there are 30 entities the signal output would be 30/4=7.5 rounded down to 7.

And of course, it would require power.
Thank you.

The sensor in rftools can do this but in a limited area (i.e. right in front of it or a few blocks distance). Will that work in your situation?

Well it's kind of the same problem that the Minefactory Reloaded's Mob Counter had, the area is too small to work with most spawners or animal farms.
I've always tried to avoid compressing too many mobs into a small area (resulting in them being on top of each other) because in my experience it creates more lag than leaving them room.
All that being said, I hadn't taken a look at the sensor yet, very interesting device.