The sliding steps on the doors will have sensors on them to detect the platform edge which determines how far the sliding step extends. A potential solution that would help in a situation like this would be to set it up so that if the sliding step reached its maximum extension without detecting the platform edge, the door would remain closed.
I did at one time manage to find the developer's manual for the IFE/Knorr-Bremse FLEX door modules (via an Indian Railways tender document cache which is sadly no longer available) and as standard it does not expose signals from the step actuation part of the sequence on it's interfaces - it simply gives a 'DOOR OPEN' or 'FAILED' response to a 'RELEASE' signal from the train control system, depending on whether the step and door have successfully completed their sequences.
IFE say in their mearketing documentation that the firmware is customisable, but it means Stadler (i.e. Nexus) would have to pay IFE to add an intermediate 'STEP AT FULL TRAVEL' response into the sequence, and then Stadler would need to rewrite their own door control code to handle it.
I believe Merseytravel are also currently paying Stadler to integrate either Stella Tracklink or a similar system on the 777s to allow SDO on the Northern Line stations south of Liverpool Central that do not have platforms long enough for 8-car working. Nexus signed a contract with Stella last year to provide SCADA-based train monitoring and the Tracklink system to enable radio changeover, so if they want SDO or CSDE, the system is there, they would just need pay Stella to extend it, and Stadler to integrate it (possibly piggybacking off the work for the 777s).
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