It's an interesting theory!
Yeah I like the idea about the veil being thin on All Hallows Eve and all...coz on Halloween, there are many theories about ghosts and spirits from the other world being more prominent in this world for one night.
But I don't like the idea about Harry dying..not only because I don't want Harry to die but also coz, All Hallows Eve will be on 31st October and HBP left us at somewhere in May/June.
So DH will pick up from around July/August. So by the time October 31st comes...the Trio wouldn't even have time to find and destroy one Horcrux, let alone go after Voldemort.
And if Voldemort isn't gone and destroyed, then Harry can't possible go n fall into the veil, can he? It would be a huge anti-climax!