Okay, I found it, seems at least undocumented in the tuner interface. At least the help says "<HIDCode 0x##>" which doesn't work at all.
If I want to send a specific HID-Code, I add a new chord, choose "keyboard" and enter
<keycode 0x##>
where ## is the hex HID-Code. For example
<keycode 0x4> is an "a" and also gets replaced by "a" in the tuner interface when you apply the config for this chord.
Regarding my specific problem, here is the following solution (the bullets are from the list in the markup)
- < = <Non-US|>
- > = <L-Shift><Non-US|></L-Shift>
- | = <R-Alt><Non-US|></R-Alt>
That's interpreted on the android side in a German keyboard layout.