Thanks for the reply Christian.
Much confusion is caused by the term "visualization" itself. Visualization is just imagining: Creating a mental image, very ordinary, nothing special about it. You can find the place where mental images are created by imagining something simple, say a triangle or a square, and build up from there.
I understand that visualization is just imagining. The trouble is that when I try to lock onto the place where visual images appear, they just disappear completely. I'll see a brief flash of the visual object and then it's just gone, not even replaced by other visual stuff, the whole inner visual field disappears from view. Sometimes I can get it back, but other times it just stays dark.
I like your idea of carefully investigating the difference between somatic sensations around the eyes and visual sensations, but again, I'd need to have some visual sensations that stick around to be able to investigate this.
One thing I've started doing is trying to do vipassana with eyes open taking the external visual field as my object. This has been good, and I sometimes get more imaginary imagery with my eyes open than I do with them closed. It reminds me of a technique they use in parapsychology called the ganzfeld. Basically, telepathic "receivers" have half ping pong balls put over their eyes with a red light shining on them so that their visual field with eyes open is a just uniform red. In this state, vivid visual imagery sets in rapidly. I should try that sometime, all I need is a ping pong ball and a red light.
Keep the ideas flowing!