Found some data on a UART channel connecting the sensor board to the bluetooth/main MCU on a Mio Alpha watch. Wrote a quick script to assist in viewing the data on this bus.

MioDecode.py

'''
MioDecode.py

Reads data from the internal serial bus of a Mio Alpha and prints the BPM to screen.

Dependences:
    -pyserial

Usage:
    python.exe MioDecode.py --port='<serial port>' --baud=<baud rate>
'''

import serial
import argparse
import sys
import time

parser = argparse.ArgumentParser(description='Consume CSV data from a serial port')
parser.add_argument('--port', type=str, default='com7', help='The serial port to open')
parser.add_argument('--baud', type=int, default=19200, help='The baud rate to use')
args = parser.parse_args()

ser = serial.Serial(args.port, args.baud)
ser.setTimeout(1.0)

def waitForStartChar():
    c = ' '
    while c != 'o':
        c = ser.read().decode('utf-8')
waitForStartChar()
start = time.time()
while(1):
    try:
        waitForStartChar()
        pkt = ser.read(6)
    except:
        print (sys.exc_info())
        break
    if len(pkt) != 6:
        continue
    print("%.3f: %02x %02x %02x %02x %02x %02x BPM: %d" % ((time.time() - start), pkt[0], pkt[1], pkt[2], pkt[3], pkt[4], pkt[5], pkt[5]))
    
ser.close()