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I just received my VIC-II-dizer, a clever hack which produces a crisp HDMI output for the #C64 by snooping the data bus of the video chip.
Other solutions typically digitize the analog luma/chroma outputs of the VIC-II or replace the entire chip with an FPGA replica.
Unfortunately, I can't test the VIC-II-dizer today because the only #C64 I have broke down last week 
I've already checked for power issues and basic bus connectivity, but I can't go any further without an oscilloscope and/or a diag cartridge. And then I will likely need to order a new IC somewhere...
I borrowed a Versa64Card from my friend @giomba.
To help diagnose the fault, he pre-programmed a flash with 4 different diagnostic ROMs which can be switched by holding the top address lines either high or low.
Of these, Dead Test has the best chance of running on broken boards: it sets the #C64 to "Ultimax mode" and doesn't rely on the KERNAL ROM. In fact, Dead Test can run with most chips pulled out except for the PLA, VIC-II and, of course, the CPU.
It works! With all chips pulled out, Dead Test starts! The funky colors are a pre-existing quirk of this VIC-II, something I was hoping to workaround with the VIC-II-dizer I bought.
We reseat the chips one by one, until... Aha! Installing the CHAR ROM in U5 causes even Dead Test to... die! 
Looks like two chips are fighting to drive the data bus lines.
Is the chip-select pin shorted to ground, causing the CHAR ROM to respond on every bus cycle? That would surely mess everything up!
Lifting the chip-select pin out of the socket and hooking it to +5V with a resistor fixes things, suggesting the ROM is actually working fine.
The real culprit is whatever holds the CHARROM chip-select signal low at the wrong time 
@codewiz could still be a broken PLA…
@root42 You were right! Check the rest of the thread for an update.