I'm running a game up in Saskatoon, Saskatchewan, Canada... I've been using a combination of a barebones MS Access database and, well, just keeping very organized notes. I'm used to running downtime for systems like Kingdom Come or Vampire, too, though. No substitute for Storyteller sweat and tears, really.
My dayjob is, however, as a Software Developer. So don't think I haven't thought about putting an application together. But for it to be something really solid and broadly usable, you're talking at least a hundred hours of work - honestly probably a fair bit more once you do all the testing and the like. And, really, for just my own personal use, is it worth that time? I could spend a hundred hours on a piece of software...that will end up saving me 40 hours of downtime-processing work over the next year. That doesn't seem sensible. And I could hack together something quicker...but the less polish and refinement I put into it, the less time it saves me. In the end, for one person, I think the balance is essentially "Throw together a few tables in MS Access or a tracking sheet in Excel, and then do all the rest of the work by hand", which, really, anyone can do.
If a lot of people would use such a program, it might be different. If I could get, say, 100 people willing to pony up $30 for a copy, well, suddenly it becomes all sorts of worthwhile. And if this was D&D, that might be the case...but I don't think there'd be 100 people who'd pay for an application to manage Houses (whether that be, like, a Java desktop application...or a PHP website dealy, where players could go and enter their turns online, and then the ST process them from a central management page). Even including tabletop GMs, I wouldn't think there'd be the sheer numbers of folks who'd want the software to justify it...
Long story short - I'd recommend just tracking it all in a spreadsheet without automation, and just set aside the time before each game to go "Okay, I've got to add this to his Resources for his Production, now go down his list of actions for him and all his vassals, double-checking to make sure he's taking the right number of actions and not listing any vassals he doesn't have, and then..." just like you would in a tabletop game. Only multipled by 25 if that's how many LARPers you have.