ummm, yea it comes out from the front, and you press it in from the front, but it would be a little easier with the timing cover off...can you replace it with the cover on the engine? Yes...just pry it out, and tap the new one in with either a dead blow hammer or soft mallet. The biggest risk you run is dropping pieces of the seal (little metal coil seal) into the pan, or possibly screwing up the seal upon installation.
Honestly, I'd probably pull the cover to replace it though...
I did it a few weeks ago. Once the balancer was off I used a pair of Visegrips to grab the flange lip of the seal. I screwed the adjustment on the Visegrips as tight as it could get it and still be able to clamp the jaws shut. I clamped the Visegrips on with the release handle facing the radiator and rocked the backside of the Visegrips against the timing cover. A short prayer for divine intervention and a big tug resulted in the seal prying loose on one side. Some more pressure on the Visegrips resulted in the seal prying loose. Since the balancer had spun and was in two pieces, I used the center part of the old balancer to drive the new seal in place. Coat the seal's metal shell with a little bit of blue silicone for leak protection.
But it turns out that I just looked at it wrong and I was pounding on the edges of the seal instead and it was fine...lol