I have a bunch of live tiles that are handy to glance at to tell me what meetings to go to - this is great, but when my phone is on my desk on charge, but it doesn't seem possible to set a different time out when on mains (or plugged into a PC) versus when it's running on battery?
Yes, and you can do this in
Settings > lock + wallpaper > Screen times out after. But not all phones supports this action. I know Nokia Lumia 800 have max time out limit of 5 minutes. Also remember this action is not specific when charging.
See how in this video.