You are correct - watchfaces can not access sensors. But, watch-apps can.
You could do a watch app, that looks like a watchface, but you must take care to not impact the battery too much (things like not updating the screen every second, etc.) Just accessing the sensor will also have an impact on the battery.
I did a watch app that looks like a watchface, but uses the Tempe Sensor (Temperature), and there I only update the screen every minute (kind of like low-power mode for a watchface). On top of that, the Tempe only transmits once a minute, so there's not a lot of data coming in for the FW to handle. And even then, it drains the battery more than a watchface.
Yes it's a good idea ! and the only way to do this... but as you said, i wonder if the battery will survive even if i check the pulse only every 5 seconds !! :confused:
