We generally don't edit BIPs like that after they've been written except to add helpful links, examples etc and other things that don't add new functionality. For this you'd write a new BIP. It doesn't have to be hard. The process is:
1) Adapt the template BIP and fill it out with your motivation, design, rationale and ideally some examples.
2) Post it here and ask Gregory for a BIP number. He will select one through some magic algorithm I am still reverse engineering ;)
3) People will give feedback and try to spot problems in your spec.
I looked at what Andreas is doing a few days ago - basically it's just an xpub string with a ?a=b&c=d query string added to the end, with a hierarchy type specifier and something else I forgot, encoded into a QR code. So it should be a very easy BIP to add.
Whilst you're at it you might want to add an HTTP POST based method, though. Web apps scanning QR codes is kind of clunky compared to just picking Coyno from a list in the wallet app and having it all auto-magically activate.