It sounds ideal for laying, as long as the trunks are thick enough. You can use a saw rather than a billhook to make the cuts in a more controlled way. We did a 50' stretch of our hedge a couple of years ago (much to our neighbour's shock as he thought we were chopping it down) That bit is now very bushy from the ground upwards but did look like a massacre for the first few months
Even then the hedge is not totally hen proof and a fox could push through, although it keeps the sheep in. I can't think of anything you could plant there which totally stops hens - we have a stretch of the thorny roses and the hens spend hours scratching around in there, impervious to the prickles
It could be that you need to both lay the hedge and incorporate a chicken wire stretch within it.