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).