Straight from their web site - you can use open face helmets if they've got the necessary standards sticker on the helmet:
Notes on Helmets...
Full Face or Open Face Helmets bearing any of these standards are acceptable;
SNELL SA 2000, SA2005, SFI 31.2 (US)
SNELL M 2000 SFI 41.2 (US)
BS 6658-85 Type A (British)
SNELL 1990/1995/2000/2005 - SFI 31.1 (US)
BS 6658-85 Type A/FR (British)
AS 1698 1974/1988 (Australian)
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