Do you find any solution ?
I have the same problem, to render 3D not really in headless mode, but when 3D canvas is not visible. I tested the LWJGLPBufferOffsreenRender but it doens't work when parent canvas is hidden.
I looked the momokofan API, but it look complicated to use only to create a PBuffer, and I not sure it will work because it seems to be the same use of PBuffer.
I read that pbuffer allow to do rendering in headless mode but I don't know how to do with JME.
I can't use FBO because is not supported by all old graphic cards.