Unfortunately, as mbt says, the helmet lights are controlled by a separate board, and the control comes through a couple of solenoid transistors. Neither of the OCD kits can do anything to help them.

I don't really know what kind of rates they're running at, so I can't really say whether something like Comet's Ultimate Optix would be able to smooth them out or if it would require a replacement board to really do the job right.

No, this is a different problem. The older Bally/Stern games just need the lamps to draw a little more current than LEDs typically do.

Here, the issue is that there is a "Chase" board that controls additional lamps separately from the typical GI or lamp matrix. The circuit on that board turns on and off fine with LEDs, it's just that the control is a bit too "crude" to smoothly control LEDs. Incandescent bulbs, with their inherent fading, smooth out the jerkiness of the controlled power.

I haven't been able to find a schematic that I can actually read, but it looks the bulbs are being powered by a DC source. Is one of the GI strings rectified somewhere?

As for the GI OCD fuses, they're very conservative values (2A SB) for F2 - F6. While I haven't specifically tested for higher current, you could increase the fuse value to support incandescents in the helmet. Just make sure to keep an eye on the MOSFET to make sure it isn't getting too hot. It's rated for 26A, but there's no heat sink on it, so it will not be able to run at the full rated 26A. Also, be aware of how much current you're running through the connectors. It's no more current than the game was originally running, but we've all seen burned up GI connectors, right? The fuse holders are rated for 6.3A, so don't go higher than that.