You need to connect mode to 3.3V, otherwise nothing will come out of luma. Though since you have enabled VGA during BBLoader installation you have to connect mode to 3.3V few seconds after boot, otherwise it would output VGA. Also, since 480p is already enabled, your monitor might not be able to handle a 480p "composite" signal from luma.
I didn't go through with this test based on what you mentioned, thanks for the heads up.
Those TP pads are not all ground. Even though some have continuity to ground on a multimeter most are a data line. Maybe use the outer ring of the screw holes on the GC motherboard as the ground. Or the ground side of a resistor. Also, your composite Via still has some mask on it. Make sure you scrape off all the green mask so it exposes more of a solder pad.
I used the composite ground based on the guide, but I'll try another location. For composite, I tried the via you mentioned, but then relocated to the edge side pad at C91 on the other side of the board and still had no signal.
I'll move ground and see what happens.
On a side note, I used the wire harness included with the screen to feed it a composite signal from an outside source, and it displayed output. So no problems with the screen itself.
Update:
So I tried everything. Different composite points, ground points, and even rewired U10 with different wire. I'm getting a solid 3.3V at the GPU via, and still don't have a composite signal.
So, that leads me to my question. I bought this Wii used, and tested it's pre-trim functionality through component cables where I had no issues. Is it possible that composite was just cooked from the get go?
Is the best next step just move on to wiring VGA?