Well that's a shame that there's a limit. That's why CHDK hack firmware and Canon point'n'shoots are quite nice because of unlimited interval shooting. Don't know about tethering with those though
I didn't exactly get how those routerboards work? Is it like you connect camera with (usb) to routerboard, and then connect routerboard to pc with wlan ?
Sadly K-x doesn't function with pentax Remote Assistant... I think K20D is last camera to operate with it. You should see
this thread one polish guy is developing a tethering solution to pentax cameras.
I tried the 0.3 version with my K-7 and was positively surprised, interval shooting didn't work for me, but it might work with your K-x. Also that's an early beta stage so it might not be the stablest around there, but could work for you!
Also with dslr there's always a risk that shutter and mirror breaks during ultra long timelapses. (My K10D did well for 55.000 shots for one of my timlapse projects. post your pictures
thread here if you're interested. Also it's sad that there's no option for a "forced mirror lockup"
-Juho