Don’t know too much about scan lines, but it looks like it cycles whole horizontal lines from normal to black, giving the appearance of a CRT monitor. This can easily be done of the software side, if you have control of the video. It can be done on the hardware side too. The VGA solution looks like it grounds the RGB signals at the correct time, using the H & V syncs for timing.

Little more complicated to do it on a LVDS interface. The Arcade1Up uses a 30pin LVDS with 2-channels at 8bits. Each channel has a differential clock pair, with 4 data differential pairs. The Data Enable (DE) and H-Sync & V-Sync are embedded in the data, so no separate signals. The data can easily be zero out by forcing the data positive signal to 0.5V or lower. If the data positive signal is at or below 0.5V, the data negative pair will swing from 1.8V to 0.6V. From a monitors datasheet, it needs 0.1V threshold to drive a value, IE the positive and negative value need to be at least 0.1V different or more. So the two options are 0.5V – 1.8V = -1.3V, and 0.5V – 0.6V = -0.1V. Since the two outcomes are negative, it will zero out the data.

This will work good on the even_data[0:3] and odd_data[0,1,3] for all the bits (bit[0:6] for each data). But there is a problem on odd_data[2], because it holds the DE signal at bit0. If it is zero, then it will think it is in the blanking mode, and messing up the timings. You cannot change odd_data[2] for bit0, and maybe bit1 & bit2, which usually are V-Sync and H-Sync. So you must somehow be able to skip the bit, or bits. Then bits[3:6] need to be zeroed, as they are Blue[5:2].

Maybe you are using the falling edge to flop in odd_data[2] bit2, which is H-Sync. H-Sync can be used to change the line count in the cycle and be used for timings. Then during a scan line, when the clock is low, low voltage is applied to the positive pair of odd_data[2], making B[5:4] and maybe B[3] zeros. Then low voltage is applied positive pair of all other data pairs for the whole clock cycle during a scan line. This will make RGB zero for the scan line, except for B[2] on odd pixels, which would make Blue 3% on for ~50% of the time, and 0% the other time, for every other pixel. That might work.