I thought I remembered someone discussing getting RSSI off of a high channel number, but I can't find any such post.
For any one serial protocol, it would be possible to add the capability of getting RSSI from an arbitrary (large) channel number. This becomes difficult when you try to do all protocols because you really need to test all those protocols. It would also be possible to just increase the number of input channels, but that can cause timing issues for PPM for example.
Here is a post from a guy that just got analog RSSI from his FlySky receiver. Maybe you can PM him. Also Google may find more such mods.
https://forum.librepilot.org/index.php?topic=4239.msg28791#msg28791You may be able to find an IBus to PWM converter and use the PWM of the RSSI.