hi roy
from my experience, creating detail in the hightlights with lith film is very difficult. It seems like it's a matter of finding the exact developing time/agitation amount so that the lith JUST reaches the right highlight density. lith film is really finicky and the highlights block up very quickly.
Ofcourse, if you're working with simply a high key image with no shadow detail then it will be much easier to obtain highlight detail.
With making the negative, flashing would, I believe, just effect the shadow detail, if anything. But if you are using an inter-possitive, flashing could work with that?
But that's just my experience. I have never tried flashing. Perhaps someone will have more promising information, which I would love to hear as well.
david