I think we are both aware that the emphasis on nimitta usage came much later. If pressed I would say that it is covered in 'breathing in with the mental fabrication' portion of the anapanasati sutta. Jhana is included in the sutta as the 8th step- releasing the mind- this is clearly ceto-vimutti, as opposed to panna-vimutti.
Practically many people don't develop clear nimittas, but are still capable of attaining to jhana. I have seen this happening. Therefore I believe the Buddha didn't persue the matter of the nimitta that much.
Focusing on a 'spot' allows the development of strong samadhi- which is quite useful- but what the Buddha is describing in the anapanasati sutta is the fulfillment of the Four Foundation of Mindfulness, using anapanasati- so it doesn't have to fit exactly into a samatha description of the progression of anapanasati (and it cannot).
I find practitioners following entirely a samatha progression, often tend to lack in a certain degree of sampajanna (clear comprehension), not knowing what is happening- just keenly concentrating.. That is not adequate to develop the perception of impermanence, as per the last tetrad of the sutta. Therefore I believe the Buddha begins practice by 'opening out' the mind a little bit (be aware whether you are breathing in or breathing out) and being more aware using an expanded scope-field of mindfulness (..of the body,mind, feelings etc, along with the breath) and finally leading to being aware of impermanence and the subsequent developments of the mind, while watching anicca.