Personally I’d avoid implicit behavior. I already have an example that highlights five lines 
I implemented a workaround for this, which uses a theme component that looks for a wrapper div containing config for the wrapped GitHub link. The component waits on the DOM modified event to know when the link is replaced with code lines and then works on the li.selected state to select. This hack doesn’t change the lines retrieved so it can’t add/remove context.
I think perhaps an explicit method for controlling the behavior is better than changing any of the defaults or trying the make the component smarter.
Conveying the config/settings in the URL itself could work but it undermines the semantic meaning of the link. If using a wrapping tag isn’t an option, I assume you’d want to at least strip off onebox-specific config.