Yes I think 5mm scorio will be very beneficial in your mix for this. I once used 10mm scorio and bags off potting mix(unsifted) to pot up wisteria and and orange tree that was ripped out of the ground for a pool installation. Both recovered very well. Both are still in the same mix two years later!
I would love to have a supply of 5mm scorio but I have used crushed scorio at 100% last season and it did not go so well.
Ok enough confusing you. 5mm scorio will be good as a filler I reckon but leave it at that. 50 /50 sifted potting mix and grit (sifted 1-4mm rock or sand). Then add 1/3 scorio.
Seeing as it is bare rooted make the mix drain well is my best bet.
The best results I have had is keeping the root ball intact and using a less draining mix around the root ball to match the clay around the collected root ball but that is for later reference

It's too bad your in such a hurry cause the stories I could tell you, Bushels and baskets of stories, hole crates full of stories. But if you can spare a moment I will tell you one story.