Latest update - I realised side-scroll mode is limited to the X axis only, which would mean I'd have to build levels with ledges/cliffs/whatever all facing the same direction. Which is sad. So I modded the engine adding side-scroll input from X/X-reversed/Z/Z-reversed axes! Here's a little snap taken out the side of the prison:

So you can be running around in 3rd person view with FPP input mode, jump out a window or whatever, and regardless of which direction you're facing or the ledge is pointing, you can land on that ledge and switch to side-scroll mode. Walk left means walk left, and walk right means walk right, regardless of the player's orientation! Small thing I know, but it adds a lot in terms of gameplay dynamic - a lot more so than other bits I experimented with like yelling into vents and knocking out lights