A crude proportional (self learning) controller is desired.
Is there anything in your post that is not solved by a Nest thermostat, other than monitoring boiler temperature and comparing that to room temperature?
And as the house temp rises close to the thermostat setting, the circulating water temperature could become less hot.
What merit is there to doing that, anyway? It seems to me that adjusting boiler temperature based on outside temperature is the best method, and (based on your description) you're already accomplishing that automatically: When it's colder outside, you want hotter water. Right?
I mean, the efficiency does improve with cooler water, but the efficiency of the system is already determined based on the amount of work that it has to do (which is dictated by outside temperature). What's wrong with that?
Lowering the water temperature based on inside temperature seems like a fool's errand: Once the system finally gets caught up to the thermostat setpoint, it would lower the water temperature. As they night (or day) gets colder, the water would continue to be cooler. Until it's so cool that it's no longer usefully warm, and then it has to play catch-up.
This will, at worst, cause temperature oscillations (even with the simplest mechanical thermostat running the circulating pump), and at best increase your time constant dramatically on days when you need your heat to be working at its best.
And in both of these cases, your family will hate you for it.
For instance, the weather is cooling off 20 degrees F over a period of about 12 hours here where I am, today -- mostly during the day. The thermostat is keeping things warm just fine, which in your hypothetical case would mean a decrease in water temperature -- when you really, really would be needing the thermal mass of all that water to be hot, tonight.