Hello,
First of all, nice to see someone working on technical drawing. I think many of inkscape users actually use it for little scientific work. Even if it's not its purpose, we must admit this software is so easy to use that we use it for everything.
When you first started to talk about technical drawing, I have been immediately thinking about real time helpers.
When you practice traditional technical drawing you need to be very accurate for each line you draw, then you use rulers and compass. For the moment, in inkscape, there is snapping (to grid or to objects) and guidelines.
I have some experience in technical part design. I use the software Catia and I think its 2d editor is worth looking at.
In Catia, when you are in path creation mode, there are plenty of helpers very useful : perpendicularity, parallelism, same length or alignment.
Let's take an example : (this is grid independent, I could have disabled it in my screenshots)
1 I start with a segment (not horizontal or vertical)
2 I want my next segment to be perpendicular to this one. When my pointer comes near to a invisible perpendicular line, it snaps to it (see http://steren.giannini.googlepages.com/screen1.png)
3 Then I want my next segment parallel to the first one. When my pointer comes near to a parallel, snap ! (see http://steren.giannini.googlepages.com/screen2.png)
4 It actually also does length comparison (see http://steren.giannini.googlepages.com/screen3.png)
5 or alignment detection (see http://steren.giannini.googlepages.com/screen4.png)
This kind of help would be very useful in inkscape for technical drawings, scientific schemes, icons but also for ordinary art.
I hope this will inspire you for your "Technical drawing Inkscape GSoC"
Steren