Launch application from the touchbar
Beside BTT, there is some free option that let you customize your Touch Bar as you like (with the customize widget they provide) and can make iterm2 appear thanks to the dock widget : Pock & EnergyBar
TouchSwitcher & Rocket are also a good call as they are only about quick launching an app.
There is also My TouchBar My Rules who claims to be just like BTT, but free ! However it's looks like it's not very user friendly (but never had the chance to try it so far)
Apple provides some simple modifications for the TouchBar — go to System Preferences → Keyboard → Keyboard and click the 'Customize Touch Bar...' button to see the available options — but none of them will accomplish what you want. You can get BetterTouchTool which will allow you to create custom touch bar items, but it is not a free app, and not (so far as I know) available in the App Store.
To set up a touch bar item that opens an application using BetterTouchTool,:
- Download, install, and then Open BetterTouchTool
- Select All Apps on the left, and select Touch Bar from the pull down menu at the top
- Click the '+' button to add a button/widget
- From the Select Trigger popover that appears, choose Touch Bar Button
- Give the button an icon and/or title, then click the '+' button to add an action
- Click the button labeled 'no action' to open the popover, then scroll down and open the 'controlling other apps section
- Choose 'Launch Application/...' and select the application you want to open
You should now have a touch bar item in the control item area that will open the app in question.
It may be possible to work something out with scripting for free; I'll look into it.