Constitution Hill resumed with a 9-1/2-length win over 2414m on the all-weather track at Southwell, which was run in the early hours of Saturday, Australian time.
It was Constitution Hill's first start on the flat and came after falling in three of his previous four jumps starts. He had won his previous 10 jumps races, eight at the highest level.
The son of Blue Bresil was foaled on March 17, 2017 but would be considered a 10-year-old by the time the Melbourne Cup is run given how Northern Hemisphere-bred horses are assessed in Australia.
Eight-year-olds Toryboy (1865), Catalogue (1938) and Twilight Payment (2020) share the record for being the oldest winners of the Melbourne Cup, which was first run in 1861.
Given most of his career has been spent competing in jumps races, Constitution Hill needs to qualify for the Lexus Melbourne Cup via Black Type flat racing and, like all runners, be cleared via Racing Victoria's strict scanning process.
Buckley expects that will not be an issue but did concede he has an unconventional gait will trotting up, which Henderson will make sure those doing the checks are across.
"As far as I know, he doesn't have anything that would be adverse to getting in," Buckley said.
"Nicky will make sure that they have a video of the way he trots…he's a terrible trotter but he seems OK when he's galloping."
Constitution Hill is as short as $15 for this year's Lexus Melbourne Cup, which will be run on November 3.