For you hardware hackers out there: there is now a custom replacement board for the SMA-Q2 based on the nrf52840 with a custom purely analog pulse sensor. If you manage to open the watch (it IS possible without breaking anything) it is quite easy to install. You'll have to leave out the plastic frame holding the battery. Firmware updates can be made over USB now...I've only flashed the current Adafruit bootlader right now and the thing shows up as a mass storage device. Not done any programming yet.
Not sure whether this whole RTC-based power management stuff is really necessary since the chip can run on really nice low power modes, but this can also be omitted in future revisions. All basic housekeeping (LDO, charger, buttons, accelerometer, display connectors, vibration motor driver...) is in place and the dimensions are correct.