Hi Helge,
What sort of shapes do you want? Can you supply an illustration? [A MGE - minimal graphic example!]
You
can use graphic files, or you can write Scheme to generate custom
postscript paths. The latter can be very complex, but is of course very
powerful. or you can use path construction commands for the stencil of the notehead.
There are quite a few examples on LSR of how to make different shape noteheads. You may find id 891 to be the simplest, avoiding postscript (which has some disadvantages in relation to SVG output if you need that).