Back with an update on one of the PS3s - just managed to fix one of them, an CECHG04, by reflowing the RSX/Geforce 7800GTX.
It did came back to life, and does manage now to boot into the system
The bluray drive is dead though - it does take the disc in and eject it properly, but I suspect the bluray side is dead, and the DVD side may be too.
Oh well. I'm not going to use it too much though, and the reason behind this is the 90nm RSX. Good god, if you guys thought the Prescott P4s and Smithfield Pentium D's were hot, you really haven't met the first generation of PS3s (up to and including CECHHxx models)
I hope the reflow does last, as it's been done at 350*C for about 3-4 minutes. I also took some preventive measures to prolong its life by removing the cover from the PSU (an german Youtuber named UltraNSC has done the same and said it's a good measure to make it last longer).
Other than that, a peek at the system version revealed a somewhat nostalgic sight: it's still on 4.31! I was expecting at least 4.78 or 4.80 with the lowest I could expect being 3.55 or 3.56 though.
It's going to be modded anyways, so stay tuned, maybe I can keep this G04's roaring RSX at nice temps. (the fan doesn't ramp up a lot like my long gone G04 did)
And finally, I'm leaving a shot here so you can see how it looks now
The top cover comes from the dead CECHH04, but the rest of the top case is the original one that came with the console.