Vase,
I would think that you have a good resource in that sawdust. I can think of a couple of ways to useful it.
First, maybe consider using it as a top dressing. This would leave the soil beneath undisturbed. Alternatively you could mix it with existing soil.
If you are concerned about the sawdust being nutrient deficient, maybe consider adding some dilute
urine and I am certain that you will have a very good growing medium. Lastly, if you feel really ambitious, you could try to grow
mushrooms in the sawdust. If the mushrooms do grow then I think you will have absolutely magnificent bedding material. I would suggest Wine Cap mushrooms for this option.
I grow exclusively in
wood chips broken down by Wine Cap mushrooms and that bedding is fantastic. I would think that if my chips work well then your rotted sawdust would be even better.
Eric