Can the righteousness of the law be fulfilled in believers?
Yes. The righteousness of the law is perfect from God's point of view, but the law had no power to enable mortal man to keep it. Under grace, Grace Age believers can have that righteousness fulfilled in them, not by fleshly effort, but because of the power of the Holy Spirit.
Les explains that the law is holy and perfect, but it was weak because it had no power to impart to man to keep it, and Israel chafed under it as a yoke. In Romans, however, the righteousness of the law is perfect from God's point of view, and now there is the possibility of it being fulfilled in us as Grace Age believers. The reason is the power of the Holy Spirit, because under grace believers have something far better.
That the righteousness of the law might be fulfilled in us, who walk not after the flesh, but after the Spirit.