On an IFR plan your are required to maintain continuous communications - period. Quite often when VHF is marginal or there are gaps in the coverage I've done exactly what the FTDK did, and there's never been any problems.
BUT:
By using another aircraft to relay position reports i.e. one that's higher with VHF coverage or another aircraft in company with HF to relay for you, there's a possibility the nice folks at CASA will hit you with an ESIR (Electronic Safety Incident Report)
In the past the Centre guys and gals have made a bigger deal about this than seems to be the case in more recent times. Two VHF "deaf" spots I deal with regularly are between Normanton and Townsville and between Longreach and Townsville. Previously I have done as indicated in an earlier post and downgraded to VFR then upgraded again when VHF comms were re-established. Recently however, Centre has offered me a sartime for my next position report - by which time VHF comms are expected to be re-established. Time is the "deaf" zone is only 30 min or so.
Did the downgrade/upgrade thing last month when crossing the Barkly Tableland enroute to Darwin. The "deaf" zone on that occassion was more than an hour.
No big deal if the weather allows for safe VFR.
I am reluctant to rely on overflying RPT to relay for me. If you can't raise someone, you will likely have some explaining to do.
Dr 