I think everyone's forgetting that this is a BD operation, not LH.
LH will offer marketing code on their service for connections but ultimately it's bmi regional operation and they would control the availability and price. LH will offer connectability where they need to, but for TXL I don't think it would make financial sense for regional to offer this sort of routing at a price that's affordable.
The current sale fares of £59 o/w are not sustainable for a 49 seat operation.