Not sure there is a hard and fast answer. On "normal" plays the R should make a mental note of those that are unrestricted. With this bizarre formation, alarms should go off in his head, and he should remind himslef that those guys spread way out there are restricted. Unless the QB is threatened, he could/should look to the front side of the play as soon as the pass is released to see what is happening, particularly with those wide guys wearing ineligible numbers (since they may not be accustomed to being restricted). Normally, he might be looking for a hold at the corner. In this case, if he is ostensibly looking for the hold, he will probably see the low block. If he has reminded himself belfore the snap about these wide 'restricted' players, he'll get this.
That is not to say the S/F can't get involved. They certainly can, since their key receiver isn't really being threatened at the moment the block occurs. If similar alarms went off in their heads before the snap, they can get this, too.
