Support for the Ecology mod would be a nice addition. It should be as simple as implementing the PollutionEmitter as an optional interface and giving config options to change the Land/Air/Water pollution produced.
It would probably be sensible to add some kind of multiplier/factors depending on boiler size or usage (e.g. amount of fuel burned) so the effect doesn't get trivialized by a larger boiler.

I've never heard of Ecology before. It looks neat! Can you tell me a bit about the mod's general scale factor: how much pollution does a typical furnace or machine generate? What levels of pollution create what effects?

I am not an expert on the mod myself, but I'll try to give a quick rundown, but the mod runs an update in specific intervals (60 seconds by default), calling a method on all TileEntities that implement a specific interface. There are some default values in the config.
The Vanilla Furnace has the following values:

"air": 100.0,
"water": 10.0,
"soil": 25.0

A lot of machinery has similar values in the 100 (TConstruct Smeltery) - 250 (coke oven) range, and much less for water and soil. It's somewhat noteworthy that the entities that are only supported by the config can't account for whether the TileEntity is actually being used or not. That's were adding actual support through the interface allows much finer control.
Some other examples of pollution are bonemeal usage (Adding 1.0 Air/Water and 2.0 Soil Pollution) and item expiration (0.0625 water / 0.125 soil).
There is a tutorial on how to implement the API.

Awesome! I'll get working on it as soon as I can.

I have worked this out after playing with ecology in my modpack, and this might not be very hard to do at all. However it would be better done on the ecology mod than on this end of things.

All you need to do is define an entry for these boilers in the Ecology config file and it seems to work.

If they included it by default then it would simply work when the two are placed together.