We rode it on the 22nd June (Monday) and it was amazing. By the end of the week, a sign had appeared at the start of the drop from Brevent saying no bikes(couldnt recall seeing there earlier in the week).
That said, i had an interesting "debate" with a french lady up there about riding in the national park (which you must traverse to get to holy trail) – but managed to persuade her it was all legit.
Still, as my previous replies, i wouldnt try it in August – at all.
ps. PSA do not attempt the descent from Bel Lachat to Merlet. I's unrideable/too steep/very dangerous for it's entirety – i found out the hard way!