How about building up a pile of snow like you would for a quinzee. If you do this once the igloo is tall enough the snow can sinter together by the time you finish the igloo. Depending on the snow you might even be able to stand on it while you finish the top. Then all you need to do is dig out the center of the pile and let your partner out.
I think that would work fine when the tunnel is needed. It would surely help being able to stand on it to finish the top. We used to pile snow on the ledge, when we dug out the door, so we could stand on it.
I use a couple different techniques to pack the top now. The most common and easiest is to use a ski pole's basket to pack across the top of the igloo. The other, which I use when I need to get up to take a look at what I'm doing, is to have a teammate kneel down on one knee and put the other leg/knee against the igloo and the other person can step up on his leg/knee. It helps it the one stepping up is light! Or, use a piece of foam pad to cushion the boot tread on the leg. When I'm building a solo igloo, it is short enough that I can lay on the igloo to pack the top. Sometimes I use a ski pole too but it is easy to see what you are doing with the igloo being so short. I build a seven foot for solo trips or sometimes I'll extend the seven foot pole settup so it makes the 7'11" igloo that is shorter than the 8 footer.
But, back to the tunnel.
I tend to have deeper snow than some and have my door below the floor. I've had the door facing into the wind, when there were high winds, with a tarp for a door and it isn't much of a factor for heat loss. I don't feel the tunnel is really needed.
For those with shallow snow, where the door needs to go up into the igloo wall, a door tunnel would help and this would work well. One would end up digging a tunnel which would make the entryway a little more work but it would certainly be simple to do.
I usually build on a hill side to help the door be below the floor level but that would mean the snow, I use to make the tunnel pile, would need to be shoveled uphill.