I was thinking a F9R could use it's ability to land to abort a failed launch. E.g if an F9R suffers an early stage engine loss, or later double engine loss.What would be needed to be done to allow it to land with a 2nd stage plus payload on top?My first thought was it needs to vent a huge amount of propellant to avoid overloading the legs during landing.
As previous posters have concluded: Asking a reusable multi-stage LV to perform an intact abort is to ask it to do something which is several times harder than a normal launch - after a failure has already made it impossible for it to do that... Saving the payload is already done for crew and might be doable for more fragile satellites, but saving the entire LV is probably only possible/worth it for SSTO (especially space planes).
...Overall, it's far from impossible but would be a complex process to develop and prove. It certainly isn't a capability that I'd want available from the outset as it would lead to delays and unnecessary expenses.
...I think planning for some of these scenarios is going to happen sooner than some might think. Basically, as soon as it becomes clear that the F9 first stage is recoverable. If there was a launch where four engines failed, SpaceX is going to want to try and recover the first stage (assuming the centre engine isn't one of them!). You'd simply keep the engines firing to burn off the propellant; launch the second stage and then try to land!
The correct way to go in the event of first stage engine failure is to abort the first stage recovery sequence to recover the margin needed to allow the second stage and payload to achieve orbit.