A strategy I've found myself using recently is to use the DSTech blade digger and just dropping through an enemy base.

What I'm thinking of making is a rocket that has essentially heavy diggers strapped to the bottom that fire in place of the retro rockets.
Something along the lines of this:

I'm going to be working on this as well but I wanted to put this out there in case I lose the interest/ability to complete this.

Move to Mod Making please.

See that handy dandy "Report" button under every post? Please use it in the future.

I think he means something that maybe instead of giving a reverse blast when you press down, digs? You can turn the digger off whenever you like and drop troops at any level.

If you have problems with sprites, let me know it, it will be pretty easy to re-sprite it.
(I also love to do that tecnic, jump + digger aimed to down)

I thought about doing this, but wouldn't it almost make it too easy to just drop one on top of a brain bunker?

Well with the amount of added velocity from the rocket falling, you could have a pretty huge hole dug in a very small amount of time.

I've found that a few 1000kg stones make a crate into a very useful tool, if you hand them to a crab in the inventory, you can add in another guy and deploy it underground.
Adding a gibthis script to the stones helps keep them from killing the ground themselves.

And that self-destruct script is...

This is what I'm going for, I know you could drop on a brain with it (and that's fun in its own way) but that's not what I'm going for. I want to be able to dig a hole, drop off a guy, and then fly back out to cause more mayhem. I'm ok with spriting, its just a matter of getting the digger part working. I think I should replace the URThruster and ULThruster with something that shoots down, but it seems that you can't change the direction. Any ideas?

If you can Lua then look at the EDV mod, it has an attached concrete sprayer on the armored car, shouldn't be too hard to add a Lua digger to a rocket.
Oh, and Shook's pack of mods has a digger drone, take a look at those, it may give you some ideas.

Probably easiest to just attach an AEmitter to the bottom of the rocket. Copy the code from the rocket's main thruster, replace the emissions with heavy digger MOPixels, and set the spread to 1.57.

Then how you will send back to orbit?

With the main thruster that's still attached? I may have been unclear, I meant to attach a new AEmitter that the bulk of code is copied from the main rocket thruster.

But to make it controllable we could, instead, copy this guy's mock-up and make that emit digger particles when you 'reverse' by pressing down.