i know how to do render, but transparency you would need to do removing background color (for example white) using shader or texture editor code.
but before, Why you cant put just 3d object into GUI node? (i understand there are a lot unique builds, but you can “auto-generate” only the ones that are visible in list) - if 3d model then you will be able to rotate it (good for someone to see all model, not one direction of it!)
about texture rendering you got something like renderToTexture in Jme3 tests.
Im using this solution myself to pre-generate map texture, to not have camera rendering it all time on minimap/map.
but like said about transparency there are 2 solutions, one would be setting background to white(or black) and clear this color in shader, second will be same on start, but clear color programically by java libs.
Also important if you want have all models in same scale centered in view, you need compare Boundings of all creations and scale maximum bounding(x,y,z) to maximum size you want for render
Vector3f bound = Utility.getBoundDistance(obj);
float max = Math.max(Math.max(bound.x, bound.y), bound.z);
float resize = 50 / max;
where 50 was units that i needed (pixels in gui in this case). its not perfect calculation, but works