LED Strips!
https://www.alibaba.com/product-detail/Arbitrarily-Cut-Ultra-Narrow-Board-1_62369796191.html?spm=a2700.12243863.0.0.6d933e5fkghAZf
They make TINY super thin LED strips –

Great, I could shave down the underside of the insert and then I could shove a bunch underneath the insert –
The problem……….

The Pixel density isn’t so good 😦
the Inside radius of the arc of the insert is about 17.1mm…see picture below…
That gives us a total circumference of about 108mm for the circle. hat gives us about 27mm for a 90 degree sweep of the arc. There’s actually a bit less than 90 degrees, so lets say that we have 25mm of length along the arc where the white dots are.

25mm of length and LED’s spaced 6.3mm apart only allows us 3 LED’s along the arc……and based on previous experiments, that won’t be enough to give a nice looking line of light……
I’ve messaged the manufacturer to see if they can do anything with more LED’s – at least double, maybe 8 for the 25mm length about 1600 per meter ;-)…who knows!
So, onto another idea……maybe a small PCB?
Looks like I can probably fit a bunch of 0603 LED’s inside!
Some quick outlines in EASYEDA
To the left is a bit of a trial to see if I can maybe get a Flexi-PCB in alongside the Insert….and another trial spacing to see if I can maybe get some degree of control of the LED’s – instead of a long block of 6 or 8 LED’s in parallel, why not 3 or 4 pairs of LED’s?
That way I can have a winking left/right status and a solid status…you could get some very basic animations going on all 4 LED’s split into 2 segments each – 8 individual lights…