In general, the invention relates to adding custom applications to multi-function peripheral (MFP) devices. In some embodiments, a proxy device is used to provide registration and routing functions to third-party applications operating on host sites. Thus, the proxy device relieves the MFP device of the processing burden associated with application registration and message routing between multiple custom applications and the MFP device. In some embodiments, user interface (UI) templates are used to customize application displays on the MFP device. By using standard templates, such as templates based on XML, the MFP devices may include minimal support for parsing the templates, thereby reducing the burden placed on the device to accommodate the custom applications.