The issue is not with ghosting or the inherent flicker in an LED. Rather, on Bally's, the controlling transistors (SCRs--silicon controlled rectifiers specificially) they used require a specific amount of current running in order to stay latched (on). If the current (amp) draw falls below a threshold, the scr turns off, then turns back on, causing a very noticeable and random flicker of the LED.
There are multiple fixes:
1) Use a big enough LED array within a single lamp that causes enough current to be pulled. You'll find that most cheap LED's draw 10 to 20 ma vs the 150 to 250 ma of a #47/#44 incandescent. If you find a LED lamp that has multiple sub-leds that increase amp draw they won't cause the SCR to unlatch. However the side effect will be extra cost and way too bright for the insert, so not doable for most.
2) Cause additional current to be pulled without creating light (or heat) to the level the SCR needs at a minimum. This is as simple as soldering a single, 470 ohm resistor across every single switched lamp socket underneath. A resistor costs 1 to 5 cents each in bulk depending on your source, and just need time to solder. This fix simply 'sinks' some of the power from the +5.9V lamp line to ground through the SCR at the socket making it appear more current than the LED uses is being drawn when the SCR is on. The nice thing is this doesn't prevent you from using an incandescent lamp as the resistance is higher than the lamp, so electricity will follow the least resistance--the lamp. The only bad thing about this is you have about 60 sockets to solder 2 wires to and makes it 'non original'.
3) Buy a new $100+ board that uses newer transistors to not require such high current draw.
I am working on an alternate solution costing much much less than a $100 board or soldering 120+ wires underneath the playfield. I have a prototype completed, tested and working, and am considering offering something up if there is enough interest. It would be completely plug-n-play and not require removal/replacement of any board or soldering or underplayfield access. The cost to fab the boards would be significant so would need to judge demand before I take the plunge LMK if anyone is interested, and I may consider this this summer.
BTW--This only affect switched lamps. GI / Backglass lamps are not affected because they either use mechanical relays in some cases or there are enough lamps on a single SCR to maintain enough current.