There’s currently just “#” that acts as a placeholder for a number. I can’t remember for sure, but I believe there was a discussion once about what placeholders should be added. The most flexible approach would allow a python expression. Not sure how safe such an evaluation would be though.
You could try to use an app handler to set the output path before a frame is rendered, that might actually work well.
#bpy.app.handlers.render_pre.clear() # clear all app handlers if needed
def set_render_filepath(scene):
# use any criteria and expression for dynamic render filepath here
if scene.frame_current < 10:
scene.render.filepath = "C:/tmp/below_ten_##"
else:
scene.render.filepath = "C:/tmp/ten_or_greater_##"
bpy.app.handlers.render_pre.append(set_render_filepath)