The Cateye LD-1000 is the best "production" standalone blinkie I know of (except that crazy DiNotte 3-watt one), and is daytime-visible to people who are in its "hot spot". The NiteRider ones are competitive with the LD-1000 from the direct rear view, but you'd need a NiteRider headlight system or else you'd have to wire it up to a 6-volt battery of your own.

I had some daytime photos from ~150 meters of the LD-1000 and NiteRider, but I seem to have lost them. Anyway, aim it very carefully so the hot spot is exactly where you want it. Use Alkaline or Lithium batteries for maximum voltage/punch (don't use NiMH, which have a feeble 1.2 volts and dim the light a lot). If the light seems to be getting dim, put new batteries into it... it doesn't seem to like low voltages very well.

Since it doesn't look like I'll need my LD-1000 anymore, send me a PM with your mailing address and I'll send you my LD-1000 at no charge If you eventually lose your mind and build a Nova like I did, then you can "pay it forward" to someone else. If you LMK early enough it can probably be on the way today.