Hi @james - as you suggested, it is sufficient to provide a custom node data JSON that defines the "mutation_length" or the "branch_length" for each node in the tree.
Indeed, it is necessary to provide the file containing this data/attributes in the last position of the augur export --node-data arguments, and in a separate file from the geographic information of the node data.
Following your approach, I am now able to display the MCC BEAST tree under the clock layout.
Thank you very much for your help.