I assume the New Holland is a NV140 or a TV6070 etc? Is the flail mower; an arm type hydraulically driven or coupled to the 3 point hitch and pto driven etc?
An excavator and a tractor are two different types of machine. If you have many miles of continuous shoulder mowing a tractor or wheeled machine is what is usually used.
If you are just doing short isolated sections of roadside then an excavator can do that job. It depends how far you have to reach and high you have to cut.
European stuff
https://youtu.be/vvjJFKvuzms (Muething mulcher) (These run off the tractor power take off, so it is mechanical power with co-joined gearboxes)
mcconnel.com (seem to have an association with alamo)
seppi - make small mulchers for small excavators - a engineer will quickly tell you how much power you have to cut with and how long you can cut for with the 25 gallons of oil in a 308.
https://www.youtube.com/watch?v=-9LLISeVDW4 (seppi mini bms 5-12 metric tonne machine made from hardox - very durable)
https://youtu.be/w_aGXxsmSw4 (Small Berti excavator mulcher)
US made
http://www.usmower.com/
tiger mower
diamond mowers (they look well made boom arm mowers) Most of them seem to be set up on John Deere tractors.
https://youtu.be/LmJwLM9Zskw
bush whacker
alamo industrial
mower max (If they used cummins engines)