In running when favourite withdrawn at post.

Don't see why not.

Betfair's rules says "In the event of a late withdrawal, Betfair reserves the right to remove the runner after completion of the race. In this case only those bets matched prior to the off will be affected by a reduction factor."

So the prices are going to look barking as the remaining field should make a near-100% book and then there will be the Fav still in - potential for 'bots to eat themselves if they're trying to beat the cross-matcher at its own game.

But if you know what's going on and you want to bet manually on the horses still in the race, I can't see a reason not to.

Why did you think no in-running?  I'm intrigued.