So I ended up with this bulb (
link). They were cheap (less than CA $15 for 2 bulbs incl shipping) and work well. The bulb is solid when riding but gives 8 rapid flashes-pause-8 rapid flashes-pause- etc etc when braking.
This means that it will probably annoy the guy or gal behind me when I am waiting at a stop sign or traffic light if I keep my brake engaged, but that is OK. It may even prevent a rear-ender.
P.S. To be clear: I have to do a bit more research but it may be that flashing tail lights are illegal in Canada, so in that case I may swap these lights back to the original bulbs.