http://www.oztion.com.au/vshops/item.aspx?itemid=4796486&tid=1994756 

Forgive my ignorance, but does E4 certified means legal for road vehicle use? Google is not helping (SHOCK!).

You shouldn't mount these types of harness to the floor behind the seat to which it is fitted. It is more dangerous for the driver when involved in a serious accident as it doesn't stop the driver from moving forward, but pulls then downwards and can break ones neck.

Camo
You raise what seems to be a very important point re fixing of seatbelts but you've left us scratching our heads as to what is necessary in what application. Could you please go into detail as to what exactly should or shouldn't be done and what specific style of seat belt your warning applies to.
Tks 'n rgds