IMPORTANT !  Years ago there was an experimental EWOCprojects build availablewhich showcased preselection highlighting as an integrated core feature.

I will make this build available again on this site in some time.  I might forget, so if you would like to use this build, let me know.  I believe it was an old 2.6x version.

Download PreSel for Blender 2.8 and higher - enables mesh element preselection highlighting 

Download PreSel for Blender 2.7x - enables mesh element preselection highlighting 


The PreSel addon enables mesh element preselection highlighting


BEWARE : dont change the name of the addon file, or the addon will stop working!

First go to User Preferences->Addons and enable the PreSel addon in the Mesh category.
Then simply click "Start PreSel" at the bottom of the Mesh Tools panel et voila!
Then click "Stop PreSel" to stop preselection highlighting.

When pressing Ctrl, one can highlight face/edge/vert paths.
When pressing Alt or Alt+Ctrl one can highlight loops/rings.
When proportional editing is on a gradient highlight will be made for everything selected and the element highlighted (this last one can be turned off).  Proportional highlighting will at the moment not update when changing the radius during tool operation, a current shortcoming in the Python API, but a feature is implemented which allows using CTRL+SHIFT+MOUSEWHEEL to interactively set the proportional radius!

This addon replaces the circle select with its own implementation:
toggle circle select on/off with Ckey
use ALT+MOUSEWHEEL to set circle select radius
select using RIGHTMOUSE, deselect using ALT+MIDDLEMOUSE
the reason for this is the user can now transform the view during circle select (so MIDDLEMOUSE cannot be used for deselecting)
also other tools can be used with circle select still operating

Go to Presel Addon Preferences (UserPreferences->Addons->Mesh->PreSel and click arrow next to it) to change options :
SelColour :  Change the highlight colour when hovering over a selected element with the picker in the panel.
NoSelColour :  Change the highlight colour when hovering over a non-selected element with the picker in the panel.
Change vertex size and edge thickness.
Set face outline drawing to on or off.
When on, you can set the face outline thickness.
Also the realtime updating of proportional editing of the highlighted element can be switched on/off (removed in 2.8 version).
Circle select tweaking: if on, use LeftMouse-drag (or RightMouse if you select with Left) to do a one off circle select
operation, use with Alt to do deselecting.  Will be slower when not in circle select, because coordinates need to be
constantly recalculated when something changes.
Show Normals :  when on, the normals of "single highlight" faces/verts will be drawn.
Normal Length :  set the draw length of the normals.
Ortho: circle zooms :  sets behaviour of the circle select radius when in orthogonal mode - changes when zooming or not
Circle select starting radius :  whenever starting the addon, this will be the default circle select radius