Just a quick update, aside from creating custom bricks, I'm also looking on modding LDD in general.
Here is my latest achievement :
I am able to mod the LDD brick palette. I can add (or 'unhide') missing parts, like the 9-11 linear actuator (only visible on Mindstorm or Extended).
For the loose parts like the ones in the screenshot, it is a little more complicated. LDD cannot load a part that is referenced by an assembly. (pre-assembled bricks, like minifigs and shock absorber) Just adding a line for the part in the palette list does nothing and if you set verbose=1 in the preferences.ini you will see in the output that LDD fails to load the part.
The workaround? simple, create an assembly for each part with only the part inside.
My software for modding LDD is doing well and will soon have an alpha release. You can follow my progress on https://github.com/Polymaker/ldd-modder