Can the iBert safe-T-seat be safely mounted behind the seat post?
Wth the childs's legs sticking forward the way the website suggests, you've got no hope.  You could easily check this by fitting the seat and trying to ride without the child -- that way you don't even need to do the clamps up tight.  You're also likely to find that the slumped-foward head rest sticks into your lower back and means that you're badly positioned on the saddle -- just what you don't want when you're controlling a bike with an extra load up high.

It's got more support than most front-mount seats but in general they're designed for you to have your arms round the child and your back behind them.  This one still has a very low back, and if the child falls asleep behind you they'll be flopping around like crazy and you won't know about it (though this happens even on much bigger rear seats as well, at least then their head can't flop backwards going over a bump.)

Most people I see riding with front mount seats have to ride with their knees very wide to avoid the seat -- and this is a wider seat than I've seen on the road.  It seems to me that there's only quite a limited set of bike/rider/conditions combinations for which front seats work: You need to be at the small end of your frame's adjustments, with wide bars, and riding on flat roads where you don't need to work hard to accelerate.