In the second case (damage), the extra dice were often required to overcome a targets SOAK. Someone in a basic battlesuit has SOAK of 10. That pretty much renders the person immune to small weapons fire (2d6) and highly resistant to even medium fire (3d6). Maybe that is in the intent, I'm not sure how scarce you expect such armor to be. We needed those exploding dice to overcome most ship armor + shields also.
I feel that damage is a place where exploding dice could remain. They help to represent that lucky shot.
OK, so all of that to was to explain why I think a small number of extra LUCK dice (as it exists today) is not going to be sufficient.
I haven't thought through this a great deal, but for the countdown I'd be thinking something to the effect of this...
You begin w/a Luck pool based on your LUCK attribute. Let's just assume that's a 4d6 pool. You can use some of those as extra dice on rolls (probably not damage, just attacks/skills). Maybe half your luck pool, rounded down, minimum 1. Probably declared before an attack roll. Each time you use it, you make a countdown check as normal and your LUCK pool shrinks according to the countdown rules.
So, if I have a LUCK pool of 4d6 and am shooting at a dude in heavy armor, I decide I need extra help to hit him so I invoke my LUCK. I add 2d6 extra to the roll. Regardless of the success or failure, after the roll I make a LUCK countdown and my LUCK pool decreases by an amount equal to the 6's I roll.
Presumably people will always use the maximum number of bonus dice they can, then, if the pool's reduction is related to the number of bonus dice they roll?
Everyone has 1 exploding die no matter how many dice they roll. They have to roll this die separately. It explodes on attack rolls, damage rolls, skill checks, etc. Basically the old exploding rule but added only to 1 die.
This solves a separate personal problem I have with no exploding dice. I always enjoy having a 'critical' option of some sort. Something to look forward too if actual luck is in my favor. Something that, given the right circumstance, lets me do the impossible.
By limiting the exploders to 1 die you should get rid of some of the weaknesses, specifically that the exploding mechanic favors those that are already favored by rolling more dice.
You could also add Careers that give you more exploding dice, either all the time or during certain instances. A sniper might get an extra die when sniping, for instance, and a reckless brawler might get an extra die when swinging his sword at someone.
Again, just some ideas.
I do like that. Folks could have a die of a different colour. I'll think some more on it.