But as far as I can see this can't be automated - or at least it can't be selected in the automation lanes.
You can turn off the entire chain, and automate that (Insert FX>Power On-Off) as a twirl-down on the track itself - but only for all of the effects in a chain at once, not individual ones.
I was talking about the effects mixer which, in 1.5, was a separate panel to the main mixer. It was accessed from a tab on the effects rack display. It allowed you to adjust the wet/dry mix for each individual effect in a chain and to also bypass each effect in a chain.
You can adjust the wet-dry mix for each individual effect from twirl-down menus. Okay, it's not a brilliant way to do it, but this does also give you a way of effectively automating the bypassing of an individual effect in a chain. Somewhat not perfect, because it doesn't disable the effect so it's still using resources, but if you lock the track, you won't be any worse off. Mainly not perfect though, because it's a
right load of fiddling about to do it!
But it
is possible... I think that most people, if they wanted to do something like that, would be more likely to use group sends to group particular selections of effects for different tracks, and automate these instead. With a bit of thought, most eventualities can be covered somehow, even if some of them aren't handled in quite the way one might have chosen oneself. It's certainly something that could be developed further to advantage.