The 19-year-old whacked eight sixes and nine fours in a remarkable undefeated innings of 156 to help the Cumming XI overhaul the McCullum XI's impressive total of 316 for eight with seven wickets in hand and more than two overs to spare.
He combined with Craig Cumming (72) in a 130-run third-wicket stand that took the game away from the McCullum XI.
Earlier in the week, Otago coach Mike Hesson stressed how important it was for players to seize their opportunity in the trial matches and the wicketkeeper-batsman will probably have a debut when Otago opens its campaign against Wellington at Molyneux Park on Sunday.
With first-choice gloveman Derek de Boorder also in good touch, that opportunity is likely to be as a specialist batsman, opening alongside Aaron Redmond.
With Bracewell in punishing form, most of the McCullum XI attack came in for some tap.
Neil Wagner was probably the pick of the bowlers with two for 52 from 10 overs.
Earlier, Neil Broom was dismissed five runs short of 100.
He whacked 95 from 98 deliveries and younger brother Darren posted 59.
Nathan McCullum (58) capitalised on a good start from his side, scoring a rapid half-century.
His 50 came off 26 deliveries and he smacked four consecutive sixes.
Warren McSkimming bowled a tidy spell for the Cumming XI, taking two for 33 from seven overs, and left-armer Craig Smith took three for 54 from 10 overs.
Another trial will be held today, with Otago expected to name its side at the conclusion of the match.
McCullum XI, 316 for eight (Neil Broom 95, Darren Broom 59, Craig Smith three for 54) lost to the Cumming XI, 320 for three (Michael Bracewell 156 not out, Craig Cumming 72, Neil Wagner two for 52).
