Mill wood

Not sure your boiler will do well on a steady diet of dry volatile slab wood, mixing in large splits could help.

Is this dried stuff or green?

Not much better way than to try some & see what happens. If it doesn't burn good (green) you might have to mix with dry firewood, if it burns too good (dry with lots of exposed surfaces) you might have to mix some not-so-dry firewood in.

I'm gonna take a guess that most, if not all gasifiers won't like a full load of very dry wood.
Even if my cord wood is to dry I will get huffing.

However, I do get free pallet blocks from a pallet manufacturer and free cabinet scraps from a cabinet shop.
I only use them in the summer for heating DHW.
I burn small batches just to bring the boiler up to temp and then shut it off until the next batch.

I buy slab wood from a pallet mill owner now and again. It works great, and it is fine to mix with conventional firewood. In fact, if your regular firewood isn't seasoned enough, mixing with the slab wood (which tends to dry very quickly) usually works well.

Typical slab wood is green but as stated dries out quickly. Free is good. Bit of a challenge to stack it so it does dry do to the flat surfaces. Easy to overfire with.

I know someone who burns mill scraps and plywood in their gasser. He told me he just stacks it good and tight together when loading it as opposed to just throwing it in. He also turns his upper temp on the boiler down to keep the fan from running full bore all the time. Seems to work for him.