I'd avoid ash, as it's too alkanine if used at any significant concentration. Chunks of fresh charcoal would probably be OK, as they are largely composed of unburned carbon, as well as a small amount of alkaline inorganics, and they will have traces of butenolide, but you probably won't get much from such chunks.
As Tripsis says, for most acacias hot water is probably sufficient. Some pyrophilic plant species require the 'smoke signal' for good germination (a lot of the leguminous vines and other 'pea' flowers fall into this category), but acacias do well with just heat. I suggested the smoke treatment just as a way of covering all bases, as you are a long way from easily trying again if your efforts don't work for whatever reason! It won't hurt, and it might help up the percent of successfully sprouted seeds.
Whatever type of water (smoked or not) you use, there is a point beyond which you can overcook the seeds. Boiling water is OK as long as it doesn't stay hot for too long, so don't use a bucket of boiling water to treat your seeds - it'd take too long to cool! To some extent, the lower the maximum temperature, the longer the seeds can remain in the water. Cutting the seed coat
can be effective in increasing germination rates, but the trick is to not go too deep into the seed coat. I wouldn't usually recommend doing this if one has not had previous experience with the seeds one is treating, especially if there aren't many to play with - it only takes one stroke and the seed is ruined. If doing this, I find that it is easier to use sandpaper to gently abrade a small patch, rather than to use a blade. For small seeds a pair of forceps helps.
Abrasion and heated water can be used together, but depending on how much of the coat has been worn away, the intensity and the duration of heat will need to be adjusted accordingly.