Ideal replacement for LEDs in current-driven strings

[mattj]

Hi all,

Just to give some context, I'm a hobbyist electrical engineer but with a background in industrial-style electrics (specifically theatre & events). My experience is mainly with digital electronics (and the high-power AC stuff) but this is much more of an analog problem!
I've recently taken on doing some repair of some china-sourced LED fixtures - similar to these.

Generally all the drive side is fairly straightforward repair - but I'm not sure what the best approach is on the LEDs. I cannot source replacement diodes - it would have to be an exact match with the others on the board which are obscure chinese parts, as otherwise it would throw off colour-mixing, so it's safer just to leave these dead - but being in a series string, I can't just bridge over the LED (at least I think not!).

There's two basic architecture:
V1: 24V supply fed into LM317 configured as fixed current source - for one particular colour, ~450ma. A variable number of LEDs (depending on their forward voltage, to maximize string length) then a low-side MOSFET doing 1khz PWM dimming.

V2: 48V supply fed straight into the string - different strings have a different number of LEDs. Low-side buck converter acting as a current source (sink?), again in this case configured for ~450ma (although there are a total of 12 converters, one per string, at varying currents). This is using a clone of a Microchip HV9910 as the controller.

V2 is the trickiest - in some cases, a single converter powers two parallel strings - and I only want to bypass a dead LED on one side of the string.

So I have four situations that I want to bypass:
S1: Dead LED on single string powered by high-side LM317.
S2: Dead LED on single string powered by low-side HV9910.
S3: Dead LED on one of two parallel string powered by a single low-side HV9910.
S4: Dead LEDs on both of two parallel strings powered by a single low-side HV9910.

Probably over-thinking this but I'm really not sure what to do for these. For S1/S2 I believe I can get away with a power resistor - but not sure of the current-voltage characteristic once they are dimmed - would this cause this string to react very differently?
I also considered using a plain diode with the approximate right voltage drop - easy enough for Red (1.75V) although getting the power rating desired isn't that easy and from my search a little more difficult for white & blue.
I can't do anything active or too complex as this has to be bodged on the existing PCB, into about 10mm max height (preferably less).

Sorry for the long post, but hopefully someone has some ideas - I've been thinking about it too long I'm worried I'm missing the easy options!

Thanks,
Matt