I love this feature in TextWrangler: it gives a sense of focus to the writing, and helps you see where you’re at on the page.
Matt
Did you try using block insertion point in Preferences -> Text Editing?
In Full Screen Mode you may also choose ‘Letterbox mode’.
I would not use it, so maybe it is not exactly what you want, sorry in advance.
Ursula
I’m not a fan of either the block insertion point or Letterbox Mode (which seems downright masochistic). Line highlighting gives you the best of both worlds - focus on the sentence you’re working on, without obscuring its wider context.
But thanks for pointing them out. 
Since SCR is based on the OSX text engine this may be very difficult and/or complex to design.
TextWrangler is based off of the BBEdit Text engine which if my memory serves me correctly is a custom text engine Bare Bones Software designed themselves.
TextWrangler is free now (free download) and is basically being viewed as the new BBedit “lite” since the old BBedit “lite” was discontinued.
Also BBEdit and TextWrangler are really more industrial type editors in that they are mainly used for coding and sofware and less for creative writing where SCR and the OSX text engine are more for creative writing and less for commercial/industrial (Coding and software development)
Have to see what Team KB would say but in my opinion if something like this were implemented it probably wouldn’t make it into SCR until version 2.0 or afterward.
The other thing is would it have to be a preference that you can turn off because many people would find this quite annoying and difficult to determine whether you are typing in a highlighted line or do you have the whole line selected and are REPLACING the whole line which could lead to much confusion.
Also line by line determination would be very difficult to code I think. (How would SCR know what line you are on if the text rewraps due to a window resize?)
Just my thoughts