The Mail article is interesting - the salary cap appears to need to include accommodation and holiday additions, but looking at the Wiggy9 Company, having over £800k "available to Directors" suggests very clearly how money over and above that intended to be limited by the Cap, can be "made available" to players - and a casual observation of the number of Internationals playing for Sarries suggests that it would be very hard to comply with the Cap by "usual means"
The key problem is that it not illegal for me as a super rich club owner, to pay, one way or another, a player for promotional duties at whatever rate I like. So long as appropriate tax is paid, its up to me if I think the arrangements are VFM or not.
But - its more than possible to regulate, if the rules are expanded to include such arrangements and signed up to. BUT it requires all the clubs to want to comply.