In your scenario the defending side are now down to eight men as the thrower-in and the goalie will have been sin-binned for time wasting too but it would be a difficult thing to monitor in a game where the clock doesn't stop, I agree. Any stoppages for subbing & injuries as well as time wasting would need to be added on to the sin-bin and it would get complicated but I still think it's an interesting idea.
I take the point about parking the bus but that wouldn't be as simple as it would with a red card. A yellow card for a keeper isn't a punishment most of the time so he's happy to take one but a sin-bin would make him think twice. Being left without a recognised keeper for 10 minutes would be tricky, the manager can bring one on from the bench (as he would with a red card) but when the keeper's sin-bin is over he's left with two keepers so would have to make another sub (assuming he's got any left). Even a centre-half going off wouldn't be straightforward, I'm sure Warnock wouldn't fancy Mendez-Laing filling in at right back while Peltier or Manga goes inside but if he makes a defensive sub he's left with a back five when the centre-half comes back on.
Sin-bins would be more of a deterrent than people think, in my opinion. I assume you made your last sentence up to back up your argument (do they really have fans at that level?). I can't find any feedback on the trial scheme anywhere but I'd be interested if you've actually got a link