Is it possible to attach an external baro to the I2C bus ?
If so, how can i check that it works and is used ?
probing the I2C buses for auxilliary baros has been fully implemented not too long ago, so you will find it in latest chibios-based firmwares.
see GND_xxxx parameters for setting the bus to probe on, what actual hardware to probe for and which of the connected baros to use as primary alt source for nav applications.
you will find all existent baro instances in your DFlogs. furthermore, if you’re using an onboard OSD, your OSDn_TEMP item will display your primary baro instances temperature reading, while OSDn_BTEMP will display the secondary instances temperature.
i do have external BMP280 baros running on all my fixed wing setups, using MatekF405Wing and OmnibusF4Pro FCs. i use them for outside temperature reading only.
cheers, basti
Thank you basti for your detailed answer.
Did i get you right: Only chibios-based firmwares support external baro ? (that would mean copter 3.5.7 does not ?)
that general functionality isn’t chibios specific. however, i can’t tell the exact dates of respective additions adhoc and, being a fixed wing guy, i’m not exactly familiar with copter firmware versions or older NuttX based code specifics.
what i do know for sure is that default external baro probing has been added to chibios in late august and respective OSD items have been added in early september 
fwiw, here’s some documentation that likely dates a while back, so it might actually be worth checking if it works on your hardware / firmware combo too:
http://ardupilot.org/copter/docs/common-baro-external.html
edit: reading carefully it states this is added with AC3.6.1…
basti.
I can’t find AC3.6.1
Latest stable is 3.5.7 and the latest beta seems to be 3.6.0rc12
Where is 3.6.1 ???
3.6.1. likely points to the next stable release that will provide these features. with the required commits added in early september, AC beta dating to early october, while the last AC stable release dates back to july, you’ll likely find that implemented in AC beta and latest firmwares.
cheers, basti.
ok, i flashed copter 3.6.0rc12 with chibios and now an external MS5611 works