That's great! There is always more work than there are workers.
You could start with these files:
src/Mod/TechDraw/App/DrawViewDimension.cpp is the dimension DocumentObject
src/Mod/TechDraw/Gui/QGIViewDimension.cpp does the drawing.
src/Mod/TechDraw/Gui/CommandCreateDims.cpp handles the buttons and commands.
src/Mod/TechDraw/Gui/ViewProviderDimension.cpp bridges the App side to the Gui side.
You might want to look at Gui/CommandExtensionDims.cpp which has functions for chain dimensions, etc.
The biggest hurdle to overcome is that the existing dimensions all refer to 1 edge or 2 vertices and have a single value. Ordinate dimensions are going to need to refer to n vertices and produce n-1 values.
Feel free to come back with questions.
PS. were you thinking about doing this in C++ or Python? If you choose Python, we'll have to expose more of the inner workings of Dimensions to Python. Probably not a bad thing in the long run.