you should be able to start navigation using the remote and then use the web interface to jump around if you want - meaning, have the ability to display the currently played content, even if it wasn't started from the web interface (upnp-cmd should list what is playing). If it's done with upnp-cmd, I expect it would work with UMSP plugins as well.
It does indeed do this already. I use upnp-cmd to retrieve this information.
I'll have to get someone to explain how to package this as an app.bin. Happy to release it even now, with the many bugs it probably has for other users.
A bit of background: Its all running as AJAX with a (similar to) REST API. I had to make my own copy of upnp-cmd since there were bugs in the seek function. The main functions were easy to write quickly, but I have spent a lot of time trying to make the interface as fast and responsive as possible. I'm sure I still have a lot of work to do. I have quite a lot of workarounds for offloading thumbnails, because with 400 movies in one of my folders, its quite easy to overload the poor little WD and crash it with too many php thumbnail requests. Now it loads them asynchronously one at a time (so as to keep the poor fella from dying) and then caches them on my drive.
Also, I have no USB at the moment. Its all running off a NAS drive, but I have used an xmount command to make it appear as a USB.
The one thing I am still having trouble with is playing videos using upnp-cmd. The vids start as you can clearly see, but the regular remote no longer responds to ffwd, rwd or any type of skip commands. If I start a vid from the regular remote, it works no problem. I think I saw something about this on the forum before but if anyone has any bright ideas I would be keen to hear them.
Screen from an iPhone 4GS:
