how deep is the root system on an inaba shidare japanese maple
We're concerned the roots may grow into our septic line
Maples in general are known for aggressive water seeking root systems, but the lace-leaf Japanese varieties like your Inaba Shidare, are small trees, actually shrub forms, that have root systems in proportion to the size of the plant. The potenial depth of the root system will be primarily in the top 12 -18 inches, but that will depend on the soil profile and layers, and where the water can be found.
I would not expect it to present a problem with a solid pipe conductor to your septic field. Any type of open system or location near the open end, or defective system will invite massive root invasion. Sewer and septic systems that are intact with modern materials and no leaks, cracks, seam separations do not usually have roots invade. At least that's my experience as a former municipal arborist, handling hundreds of issues with sewer pipes and trees.
Another thing that happens with trees and pipes of any kind is that as the roots grow around and under the pipe and expand in girth they can dislodge, displace and crack a pipe in time. I don't see the laceleaf maple as having that capability, but be smart and don't locate the tree in the immediate vicinity of the system if you don't have to.