[ravenstine]

Same, honestly, though not specifically about chest straps. Extremely primitive but effective heartbeat monitoring can be done with a cheap SpO2 monitor from Amazon and reverse engineering the Bluetooth LE signal. You won't get a true beat, but one that's more of an approximation. I know I have some C++ code lying around somewhere for this.

[numpad0]

Could be done with a webcam even. It's known that facial skin color is affected subtly by pulses. There are plenty Python codes for that on GitHub.

[kulahan]

Even full-blown OEM heartrate monitors seem to be relatively ineffective (at least, in my experience this is true when talking about watches specifically). I can't imagine this would do a great job, would it?