public class Base<T> : NetworkBehaviour
{
    [SyncVar] public bool syncvar;
    public Base() { syncvar = true; }
}

results in the following error:

Modifying a [SyncVar] from another assembly is not supported. -- both assemblies printed in the error are the same assembly

despite the error, derived classes do seem to be working for my use case, the syncvar is set once in Start() on the host and connecting clients somehow see it, I guess because the value is still being serialized even though the setter isn't working