Dear Jed,
I used to train the attention in a similar way and discovered, as a by-product, that the sense of self very easily dissolves this way.
Also it is very surprising to notice that there is a huge difference between focused attention and unlocalizable awareness. Like, the attention being an extension of perception, a tool serving 'me' and awareness being, well, just being. Not even 'there'. Even without discernible objects.
It seems that experimenting a lot like this stretches the sense of 'me' to the extreme, like an old rubber band. At some point it just looses the ability to contract back again and relaxes into, eh, simple awareness. Which is sort of totally plain and natural.
Or... it stretches to the max and contains all. Nah, nice try, but not true given that all known stuff dissolves into not-knowing. Close and dilligent examination leaves no trace behind. First concepts, then perceptions, all gone. Just a blank screen of awareness, for the lack of better word.