I've read this thread and I'm not sure I understand all the points of view. I believe the following are facts, correct me if I'm wrong:
- A "Regular" receiver puts 4 to 8 channels into MultiWii
- Serial Sum PPM, Spektrum, and Futaba SBUS all put more; 12, 12, and 18 respectively
- The received data for the extra channels are updated correctly in the rcData array (and all the arrays and code that lead up to it in the rx.ino module)
- No part of the Multiwii code does anything with more than 8 channels as written at this moment
And:
- Hamburger made a change to MSP code in Multiwii to send whatever length of data (8, 12, or 18 channels x 2 bytes each) was compiled. There is/was no associated change to the Processing GUI
- I tested that change to ensure it broke nothing in the Processing GUI
- It didn't break anything in the Processing GUI; the MSP protocol, when presented with bytes that the individual 'case' statement doesn't read, 're-syncs' itself to the next occurrence of the header; no harm
- I don't know if this would break any other GUIs, like the Android one, or whatever. If it does break them... then they are not very robust!!
Hopefully, that's a good recap. Now my opinions:
Based on the second group of facts, I personally do not see any need for any other changes to the MSP protocol. In particular, there is no need for a message that says how many channels are sent. The header length of the message containing the channels themselves already sends you this. As such, I don't even really see Hamburger's change as a change to MSP. Then length of message 105 should never be assumed; it should be parsed. That's why we have headers...
And... Whatever we decide to do with the GUI to allow it to display more AUX channels, dynamically depending on the content of message 105, whether it's change the size of the whole window to gain more room, or put a scroll bar by the channels, or change it to two (or more) "Tabbed" windows... whatever we decide to do as a group, I'm willing to do it. In processing.

Danal