Ron,
In the MA 2008 MoM's (GCR's too some) Chapter 18, Classic Motocross & Dirt Track, para 18.1.1.1 states; "An approved and correctly fitting helmet which must: a) Carry the Standards Association of Australia "AS1698" label, or b) Be approved under Rule 01.69 & 01.70 of the Road Racing FIM Technical Rules {see Appendix 1]".
Nowhere does it mention "full-face", open face helmets are available which meet the SAA 1698 standard and are therefore legal for Classic Motocross, in fact you are legally allowed to ride in open motocross and supercross events with an open face helmet as the helmet rule is exactly the same. The only MA type of racing that full face hemets are compolsory is road racing, (although you can use an open face helemt in historic road racing).
I have been wearing an open face helmet all this season in the SA reliability series and I will admit that I get strange looks because I am the only person not wearing the obligitory flash-as-Michael Jackson lairy coloured full-face hemet.
CJ