I stumbled upon this TAILDUO-50 CycleBeam(TM) TAILDUO-50 Aluminum Bike Tail Light

It's probably not super bright, but the idea seems good, and would be great to be applied to super bright lights, although it seems to only have seatpost mount, which would be useless for me. (Notice it's aluminum instead of plastic)

Hard to tell from the photos but if the lights pivot slightly the rig might be mounted with a reducer bushing elsewhere on the frame or a rear rack and still be visible from behind.

That thing looks as bright as a firefly. Seriously, it runs on button cells. It's either barely visible or it has a runtime of 5 minutes.

If you are not hanging stuff off the saddle why not buy a seatpost with the Lights In It?

Built-in Bicycle Tail Light - LightSKIN

Assuming that you don't want the seatpost you already have, that you don't want to buy a new seat post just to get a tail light, or at least that you don't want to drill holes in the seat post you have that you count on to keep you from catastrophic failure while riding. Or, that you don't want the light that goes in that LightSkin seat post, or that you might prefer to upgrade later even after this product fails in the marketplace.

But, if you're buying a new bike, aren't particular about your tail light, and will not use a saddlebag or rear rack, it looks like a useful option.

[edit: and also apparently, assuming that you want to remove and replace your seat post when you need to change the batteries.]

Because I have a carbon seatpost.

Also, how do you charge that thing? Do I need to haul a charger out to the garage, or do I need to get out an allen key and remove my seatpost every few days to charge it?

Also, a single taillight is never sufficient, because your life depends on having a working light, and you can't tell while riding if the taillight is still working, so you'll need a 2nd one anyway, at least on the helmet.

I considered Brompton version.. there you just push the seatpost down as usual for the fold..
the bottom of the frame seat tube passes behind the BB and is Open ...

but I already had fitted a Hub dynamo and replaced the M3L reflector with a wired in B&M
rackmount taillight..


Where I live, now, is not as dangerous as where you seem to be, from your description ..