Support Taranis Smart Port sensors as data sources for pixhawk/ardupilot

While I personally care more about planes, this really would be useful to all vehicles. @magicrub this is what we discussed in person, just writing it down so it doesn't get lost.
Sadly, 18 months ago instead of filing a request I actually tried to do this myself, but while I had some success listening on an S-Port bus via this method
(except on the left I used a serial-USB converted and sent it to my laptop and on the right I had sensors on the bus).
I used 3 serial to USB converters and 2 of them at 57600bps never got the signal, while the 3rd one was tuned slightly differently and was able to get the data, but I never was able to generate a packet to interrogate a sensor so that it would return something. No data I sent apparently made it onto the bus despite my having the FUL-1 converter and the SPC, so I had to put an X8R on the bus and rely on it to interrogate the sensors and listen passively.
I then ran out of time and testing equipment to see why things weren't working so, I'm opening this so that others can have a look.

To remove confusion, https://github.com/athertop/MavLink_FrSkySPort/wiki
is great, but it's meant to send your ardupilot telemetry through a Taranis radio link and display it on your remote control.
For the same reason, this request is opposite from https://github.com/ArduPilot/ardupilot/issues/1587 which deals with converting mavlink data into a taranis S-Port stream and pass it through a taranis radio link to a taranis RC remote.

This request is totally different, it is to leverage some frsky sensors and use them as data sources for ardupilot. For 3 reasons:
1) some of that data could be used directly by ardupilot
2) some can be passed via mavlink to be displayed by an OSD and/or logged on the ground
3) this also allows using Taranis sensors on a non Taranis link (actually in my case I have a Taranis but I use LRS, so I can't use the sensors either).

The ones I'm thinking of are:
http://alofthobbies.com/frsky-sp-flvs-smart-port-lipo-voltage-sensor.html
=> this is mostly useful for people who would like to monitor per cell voltage for batteries that are not balanced. This is relevant to https://github.com/ArduPilot/ardupilot/issues/6285

http://alofthobbies.com/frsky-sp-rpm-smart-port-rpm-and-dual-temperature-sensor.html
This one is much more useful in my opinion: this allows adding an RPM sensor to any existing motor and ESC. Even in my case I have a castle ESC with RPM signal out and never managed to get it work with the RPM sensor feature on AUX4/AUX5 that was added in ardupilot, but with this frsky sensor, I get RPM right away, no problems.

Start by reviewing the linked MAVLink FrSkySPort wiki and the linked issue about converting MAVLink data into a Taranis S-Port stream, then compare them with this request's opposite direction. Investigate how the FLVS and RPM/dual-temperature sensors could provide data to ArduPilot. Done means supported Smart Port sensors can supply usable ArduPilot data, with the scope and vehicle coverage clarified.

Written by the indexing model from the issue text.