While you can use polo wraps, if they are not wrapped correctly they can cause injury. They can also come undone if they are not properly secured. The velcro alone is not enough to hold the securely in place.
While polos do offer some protection from rubbing, they offer little to no support. You would be better off with either PC or CE boots on all four legs.
The professional choice boots are the best
I would consider SMB boots, but if you can't afford them, polo wraps are your best bet as long as they are wrapped properly and are very secure.
Polo warps some times gets undone while running
I agree with jasmine, barrel racing puts a tremendous amount of strain on the horses tendons, so your best bet would be smbs on all four legs.
I find smb boots way better for barrel racing , they are tougher and provide more support :)