Leaving aside the why's and wherefore's of this for a moment, I think it can be taken as read that under certain circumstances/events leading to over-pressurisation of the cooling system that coolant is being vented to atmosphere via the little hole in the top of the expansion cap. As this can, and it appears does, result in coolant being blown into the face of the rider then I seriously believe that this urgently needs bringing to the attention of BMW. By that I don't mean a quick word with someone in your dealers workshop but a formal letter/email bringing a serious 'safety issue' to the attention of BMW HQ. There may well be instance where the rider doesn't have the visor closed, standing at traffic lights etc where coolant being blown into one's eyes results in chemical contamination of same. Or, if on the move, a sudden and unforeseen flash of liquid on the visor could result in a serious accident. This is obviously a failure in design and the remedy to it is quite simple. If you think back to radiator caps fitted to cars (pre expansion tank days), when the spring on the cap lifted as a result of over-pressurisation of the coolant system, the pressure was also vented by a hole but in this case the hole had a short length of rubber tubing fitted to it that pointed in a downward direction. In my opinion the current expansion cap could easily be redesigned that either incorporated the rubber tube method or, a pre moulded elbow could form part of the cap with it's opening pointing downwards - much like the elbow fitted to batteries. However, if this matter is not brought to the attention of BMW themselves then nothing will change and perhaps one day we shall read on this forum of another such event but without the happy ending.