I've heard from several more people who have tried the highlighter with pretty good success.
Continuing my own experiments here I've had over a month of 100% success with an array of monofilament stretched between two wooden boards outside the window. The monofilament is suspended a couple of inches in front of the window in vertical lines about three inches apart. It is very inconspicuous from inside, and has worked perfectly under some very busy conditions. I have some other experiments lined up to try, including a more streamlined version of the monofilament.
I also wanted to pass along a very promising suggestion from Diane Salter of Canada, who says:
Attached is a photo of something I've been using on my windows which is very effective. My house has windows totally covering front & back. Most of the windows I netted (as I have wooden frames) but that was impossible on sliding doors & the very high upper windows.
This is the clear (with patterns) wrapping that stores use for gift packages. It sticks by static (on the inside) & can be removed easily by just pulling it off. I found that the light coloured patterns work best. It makes a visible barrier & doesn not block your view out the window altho not as beautiful (or deadly) as clean sparkling windows. It comes in big rolls, around 40" wide & a roll will do lots of windows. My house has 36 major windows + 2 sliding doors & I had plenty in one roll. It needs replacing every couple of years as it starts to lose its grip.
It really does work. I rarely have bird strikes & when I do it's only a glancing blow. Currently I have 80+ redpolls, 100+ juncos & numerous other species at my feeders.
Finally, to report some negative results, about four weeks ago I tried drawing vertical lines on the outside of the glass with a black permanent marker, but that had no effect - still lots of collisions. It might work in some situations, but not here.