FWIW cars pull up to and through junctions at ~4mph often; as I said before the exact angle just changes the way in which you are less likely to be seen.
I am pretty sure that doing that speed just pulling up to the junction is not sufficient to cause a serious "constant bearing" hazard - a car would have to be at 4mph for the whole approach and if they start the approach at, say, 7mph then the bike would have to be going faster than most of us would ride past a junction with an approaching car. Sorry to say it feels like clutching at a straw to replace the previous geometry claim and bolster a prejudice arising from some past bad lamp/mount combo.
it only needs your light not to be very visible whilst the driver of the other vehicle is looking in your direction
. That might only be a small fraction of the time they are pulling up to/out of the junction. I've even caught people in cars not looking at all
before pulling out in twilight; these folk presumably do the same thing in darkness too (but I can't see them) and they are essentially relying on bright lights appearing in their peripheral vision. These people are idiots obviously but they and others are not always going to see a partially obscured light.
Windscreen pillars are sufficient of a problem to cause exactly this kind of accident (and thick ones can take 'for ever' to get used to); lights that are not properly visible because they are obscured by bike wheels are plenty enough too. Don't take my word for it; upthread Axle Knutt reports that, in hindsight, this may well have been the cause of his accident.
I've spent plenty of time riding motorbikes as well and when you see a vehicle at a side turning it really pays to keep an eye on the driver of that vehicle; in most cases you can tell if they have looked in your direction and/or they have seen you or not. If you can't see their face because it is obscured by the windscreen pillar as they approach the junction (i.e. the constant bearing problem), you are to be concerned. Its a widely known problem amongst experienced motorcyclists and you are just setting yourself up for the same kind of accident if you insist on using a light that is practically invisible from certain angles.