The two Frenchmen are in competition with three internationals from the Irish province of Leinster: third lines Caelan Doris and Josh van der Flier as well as winger James Lowe.
Alldritt will meet the Irish in the European Cup final on May 28 in Marseille.
Dupont, defending champion, was eliminated with Toulouse by this same Irish province in the semi-finals (40-17).
The captain and star of the Blues was named best European player last year, a first for a French player.
The name of his successor will be revealed after the final.
An initial list of fifteen names was narrowed down to five by an expert jury of consultants, including Frenchman Dimitri Yachvili and South African Bryan Habana, and a public vote.
La Rochelle will play its second European Cup final in a row, its third European after its defeat in 2019 against Clermont in the European Challenge.
The Maritimes have yet to win a title.
© 2022 AFP