I/O forwarding is a well-established and widely-adopted technique in HPC to reduce contention in the access to storage servers and transparently improve I/O performance. Rather than having applications directly accessing the shared parallel file system, the forwarding technique defines a set of I/O nodes responsible for receiving application requests and forwarding them to the file system, thus reshaping the flow of requests. The typical approach is to statically assign I/O nodes to applications depending on the number of compute nodes they use, which is not always necessarily related to their I/O requirements. Thus, this approach leads to inefficient usage of these resources. This paper investigates arbitration policies based on the applications I/O demands, represented by their access patterns. We propose a policy based on the Multiple-Choice Knapsack problem that seeks to maximize global bandwidth by giving more I/O nodes to applications that will benefit the most. Furthermore, we propose a user-level I/O forwarding solution as an on-demand service capable of applying different allocation policies at runtime for machines where this layer is not present. We demonstrate our approach's applicability through extensive experimentation and show it can transparently improve global I/O bandwidth by up to 85% in a live setup compared to the default static policy.