So what are the options? Sit at the holding point with your laptop switched on?
The procedure is in
AIP SUP 7/2008.
8.1 ... If there is no ATSU at the departure aerodrome, or the ATSU is not connected to the AFTN, the pilot is responsible for ensuring that a departure message is passed by one of the following means:
(a) AFPEx (if the FPL has been filed via AFPEx) then to activate the FPL and enable the DEP message to be sent to the appropriate addressees, arrangements should be made for a 'responsible person' on the ground to telephone the departure time to the Helpdesk;
(b) FIS Exceptionally, the FISO at the ACC will accept departure times on RTF from pilots who have departed from aerodromes where there is no ATSU, or it is outside the hours of operation. The pilot is to request the FIS to pass the departure time to the AFPEx Helpdesk through which the FPL was submitted, for input. ...
I would have thought that FIS would be fairly well switched on to the last bit now, but your experience indicates at least the potential for their misunderstanding.