Also, I usually use an old t-shirt in between the the mallet and the t-molding. This prevents most marks and such.
That's funny. Digging through my dad's old trunk (1950ish??) I found a mallet that could be taken apart and reconfigured. In it's current state, it was sheathed (for lack of a better word) with an outer shell of leather, followed by a layer of fine sawdust, followed by the wood mallet head with a heavy metal (lead?) core. There were other parts, such as another leather cover sans sawdust and a metal plate and a few other parts. I have never seen anything like it since and I honestly can't fathom what it could be used for unless it meant to be used on finished woods. Pegs perhaps?
Anyhow, I digress. Since you're (@OP) is already using a rubber mallet, it might be too small of an area you're striking. Try spreading the force out a bit. An old trick would be to use a shaped piece of wood. I would select a nice piece that's nice and thin (1/2" or thicker), wide on one side about... 4" or so?, narrower on other maybe 2" or so.... Ideally, flat on one side and curved on the other, but that would be impossible to find. Then round off all the edges using your router so you don't any sharp edges. Smooth it out with a bit of sandpaper to avoid pointies. Maybe wrap it in a white cloth (an old T-shirt or a tea towel). You place the shaped wood where you want the force to go, then strike the wood with the mallet.
The idea here is that you're shaping the force applied by controlling how much force is applied over how much area. You can position the wood long-ways for long stretches or short ways for tight turns. With a little practice it'll become pretty easy. I've done this a bunch of times using a "sacrificial" piece of wood when I wanted to minimize any damage to whatever I was striking.
6" or so chunk of 2x4 to spread the malletizing out?
Damn, I had to fight with the kids and you beat me to it.