TXL will provide special assist if the airline requests it:
http://www.berlin-airport.de/de/reis...gier/index.php
So you need to add the service to the reservation for the AGP flight. No need for a certified disability, just adding a mention will do. The deadline for such notification to the airline is 48h.

LH usually allows a companion to get a gate pass so the pax can get to the gate, I guess in TXL that could be available, but not very useful.
http://www.lufthansa.com/de/de/Tipps...isende#ancAbT2

AB also offers this: http://www.airberlin.com/de-DE/site/...et_lid=6760031