Polos are not appropriate or safe for wrapping a horse that will be standing for an eight hour trip. They should only be used when a horse is being exercised. Even then, if they are not wrapped evenly, tendon injuries may result. They offer virtually no support to tendons, and little protection from blunt force injury, so there is no benefit to balance against the risks anyway.
Thick padding should be against the legs and wrapped over that with standing wraps which are of a different tensile strength than polos, both of which reduce the risk of producing bandage bows. Polos can too easily become bunched, wet, loosened, or uneven and cause serious consequences.
Going with no protection poses less risk than using polos, so if you can't provide appropriate standing wraps or shipping boots, then the legs should be left bare. I would have all shoes pulled before the haul to reduce the risk of injuries.
If you can find accommodations to stop along the way and safely remove the horses from the trailer to move them around to exercise their legs, clean out the trailer floor and assure it is not wet and slippery, that would be best. I don't recommend doing that unless you have contacted stables or other facilities with fenced areas available in advance and arranged to be able to secure the horses when they are unloaded. I've seen and heard some horror stories develop on long hauls when horses panic or escape at rest stops, or refuse to re-enter the trailer, and so forth, so keep that in mind.
Don't use breakaway halters for the trip. Use sturdy halters with either no ties or breakaway ties, and have a sturdy lead handy for each one so that if an accident happens or for any reason you need to quickly unload the horses you can just attach the lead, release the breakaway tie, and keep them under control when you unload them. Breakaway halters are worthless in an emergency.
Long hauls have been clearly linked to development of conditions like colics, laminitis and gastric ulcers. Having grass hay they are used to from home available in safe hay bags or mangers to continually chomp on can reduce stress and avoid metabolic issues that may prompt these disorders. Don't allow them to graze roadside grasses which can be contaminated, and if possible bring water from home to offer along the way.
Good luck on the trip.