For just HR the bangle recorder works just fine.
which apps do you use for ECG analysis, Jukka Happonen's?
Yes, I use this for recording ECG on the go.
Locally I use a python app that gets all the data streams on a desktop computer, shows stores and analysis it there. The communication part looks pretty simple "just" initializing and receiving Bluetooth characteristics. The basis was from a blog post example. That link doesn't exist anymore though - just checked. There were a number of options at the time, so I think you should find several examples in the net somewhere.
To me that looks all pretty basic BLE stuff (without being an expert) that I have seen similarly in bangle sw. So I guess it would be easy to write an bangle app for ECG and ACC, maybe even stop/start offline recording in the sensor.
For ECG/ACC maybe just expanding BLE Heart Rate App a bit with additional characteristics.
Is Polar's SDK open source?
Yes there is a open SDK for Android. Its not complete though. All basic stuff is in there, including downloading the internal memory (H10 can record HR without device, not ECG though).
Could EGC and the 2 Bluetooth devices support be implemented with open source?
When I checked last I found no way to activate the 2 connection feature open source, but that may have changed (approx 2 years ago). As far as I understand it can be used open source once activated with proprietary app.
Any source of documentation would be appreciated.
Just google a bit. As said my original source is no longer online, but I could find a number of projects on github immediately, including swift or ROS.