Ah, there's the problem. The plastic you are trying to dye won't take the dye.
A lot of plastic won't take the rit dye. If it could then it would have been easier and cheaper to keep the Hailstorm from 2010 convention set "in house" and not shipped off to a professional to be painted.
Your Firebat idea won't work. The plastic won't take it. You also run the risk of warping the plastic.
Same goes for the bodies of the 25th anniversary figures. The arms and legs are softer rubber and will take in the dye much easier then the bodies. I ran into major issues with the coyote trooper back in 2011. At first I couldn't find a dark brown so tan was the best option. I did 1 figure in tan and the whole figure came out perfect. When moving forward with production I tried to do a batch of 10 and ran into huge issues. The arms would take the dye but not the bodies due to the increase in volume. I tried more dye powder but had no luck.
This is why I had to hand paint all the Coyote Troopers by hand and that ultimately resulted in me pulling a muscle in my neck / head giving me daily migraine headaches and why my doctor told me "Dave, stop painting army men". After about 1 month and $350 of Physio I was right as rain, but that was the main reason I had to stop production on my end for the 2011 set.
Back on topic, it's easier to go black. You can cook them longer in black and what doesn't take you can paint. Just keep the temperature low when you do it or else parts will warp and keep your ARAH bodies attached.
25th anniversary are harder to pull off so it's easier to stick with ARAH figures.
I've never had a vehicle rit dye job complete successfully, or at least nothing I was willing to post on line with the exception of rit dying a vehicle you chromed. Chromed vehicles LOVE rit dye. Look at all those SNAKEs I made back in the day.
Martin, if you really want to make something cool I suggest getting a Firebat chrome and then rit dying it. If I were still in the game I would chrome a Firebat and then rit dye it blue. That would look professional and be something I don't think anyone else has done.
Good luck!