So far I have found that you can change which corner to send the notifications too, but not shift it inwards onto the central monitor, yet.
By editing
Desktop popups ->
steam/resource/styles/steam.styles - Lines 165 to 171
Ingame overlay popups ->
steam/resource/styles/gameoverlay.styles - Line 10
steam.stylesCode:
Notifications.PanelPosition "BottomRight" // Can be "BottomRight", "BottomLeft", "TopRight", "TopLeft"
Notifications.PanelPosition "TopRight" [$OSX] // the dock is on the bottom for OSX, so instead pop top right like other apps do
Notifications.SlideDirection "Vertical" // Can be "Vertical", "Horizontal", "None" and controls slide effect
Notifications.FadeInTime "0.45" // Controls the time it takes to slide/fade into view
Notifications.FadeOutTime "0.45" // Controls the time it takes to slide/fade out of view
Notifications.DisplayTime "6.0" // Controls the length of time at steady state after fade in and before fade out
Notifications.StackSize "6" // Controls how many panels we will stack before background queuing
overlay.stylesCode:
Notifications.PanelPosition "BottomRight" // osx has toast in the upper right, but in-overlay should not.
So it is possible to change which corner is used by setting "BottomRight", "BottomLeft", "TopRight" or "TopLeft".
However, as Abram points out
some games override that setting and other games re-write that edited file.
Each of these 4 locations/values must be defined somewhere but where ive yet to find out.
The cogs start turning... ;)