The LEDs in the controller are 0603, not 0805, though they can still be used.
Just use the 'updated' spots at the stick, it's a regulated 2.8v and is enough to drive most Blue LEDs.
At present there are 4 versions of the actual PS button, that I'm aware of anyway, and an LED behind any of them will still light it up. They've just frosted them more now and the white/silver one is only found in the Silver controllers.
The thing that makes it difficult is the rubber pad behind the button. On the older version controllers it has a frosted/white diffuser that would let the light thru, even though there was no LED, but on the newer version controllers it's the same color as the rest of it and needs to be cut to allow the light thru.
If you're really careful and take the time you can use an Xacto knife, some hot glue and a piece of white paper and get pretty close to duplicating the diffuser. Actual Diffuser in this shell....and then replaced with the Hot Glue and Paper Diffuser
Getting it wired up to where it blinks while charging and is on the rest of the time is going to make for putting a lot more in there than just an LED and a couple of wires, in fact you'd have to really get into it and figure out where to tap off something in there since the actual status LEDs do both and are controlled by the MCU, and since there are multiple board versions that's just a mess to even think about doing it.
One thing you could do though is to install 2 LEDs under the PS button, or use a Bi-color one, and wire the other one up to the USB 5v spot, it's TP1 on all versions of controller. It's not going to blink, or even let you know the controller is done charging, but you'll know the thing is plugged up to the USB cable and has power.