My farrier, the World's Greatest, confirmed that it was definitely Speedy G's right front that was causing the lameness. He also trotted him out on the asphalt and verified that the lameness was worse as the surface hardened. He pulled the shoe and discovered a very small pebble wedged between the shoe and Speedy's hoof. He cleaned up the foot, replaced the shoe, and had him move out. The farrier thought Speedy looked more sound, but he was still lame.
I stopped by after work and was relieved to see Speedy goofing off in his stall. My barn owner said that once the farrier left, Speedy was rearing and doing circus tricks; he obviously felt better. I took him for a hand jog and am certain that he is hugely improved. He was nearly sound at the trot on grass, but still head bobbing lame at the trot on asphalt. I also put him on a very long lunge line in the arena where he enthusiastically (and voluntarily) bucked and cantered for several minutes. He hasn't even wanted to walk this past few days.
Having ridden endurance horses for many years over pretty tough footing, my experience tells me that when a lameness presents itself on hard surfaces, the lameness is generally related to a tender foot. Abscesses don't usually improve so rapidly simply by replacing a shoe. He may abscess as a result of the pebble being wedged in there for several days, but I do not think he has an abscess right this minute. I am pretty sure the pebble was the source of his discomfort.
I'll just have to keep my eye on him over the weekend. I expect the lameness to fade over the next few days, but just in case it is something else, I know my vet will be able to help me sort it out.