Actually, yeah, this should be quite possible with the magic of pistons... I'm not entirely sure how it would need to be wired up, but I can't think of any immediate reason why you couldn't surround the portal with pistons, then push the victim inside and hold them long enough for the victim to be teleported. Devious idea here, I'll have to make one of these things too... 
The problem here comes in, at the following:
1. Tick delay:
1a: How many repeaters set at how many ticks, to make it so the pistons that push you into the gate does it almost immediatly, giving no time for the player to jam the pistons.
1b: a delay between initial activation, and deactivation of the pistons so they rest, ready for the next victim.
2. What's stopping them from just turning around, and heading back? well, two possibilities come to mind:
2a: No blocks surrounding the gate in the nether, forcing them to have to take time to set blocks themselves, if they brought any.
2b: Forcing them to set their own blocks gives enough time for ghast to blow them off the gate and into lava.
2c: once 1 player sets the first block, it becomes easier for others to pop in and out. But the ghast should see a stop to that, unless they slowly build a shelter around the portal.
Once they get back, they may grief your house, which doesn't sound fun.
Edit: question, does anyone have ventrilo?
