This release came from Historic Racing Club. It relates to events where VCC has involvement, however I am reasonably confident that a case could be argued for other occasions. Belts angles for HANS are mandated, using others may make the device worse than nothing.

"With regards to the trigger points that we use to implement the use of FHR here at HD, here is a summary of how we will apply these for the VCC events for the 2018/19 season:

if the vehicle is fitted with a minimum of a 4 point harness and has a high backed seat which constitutes a head rest then a tethered FHR device must be used
if the vehicle is fitted with a minimum of a 4 point harness fitted but when competing in period-build uses a low backed seat then a foam donut is required
for all vehicles fitted with a 3 point (lap and diagonal) belt system a foam donut is required
for all vehicles which, in period, are not fitted with any form of harness or safety belt system, the use on a neck support device is not required
please note that when and where a foam donut is required, this must be made of a fire retardant material meeting either FIA or SFI requirements".

There are many people that use FHR with seats other than the FIA specced ones, eg Can Am, F5000 etc. It would pay to get advice from a ruling body, in NZ MSNZ, as their requirements frequently (at least in the past) differ from FIA.