My reading of the docs is that when fetching/pushing from a SHA-256 repo, all objects are referred to (in the packfile fetched) by their SHA-256 names — then, when fetching, you locally compute the SHA-1 of each object for the translation table.
Presuming there’s some validation that SHA-1 names are unique, then that should be safe — the only way I can see one could do a pre-image attack is either fetching from a SHA-1 server (because then you don’t get the SHA-256 object name), which requires a second pre-image attack on SHA-1 (known to be feasible); or by having a second pre-image attack against both SHA-1 and SHA-256 simultaneously (and SHA-256 is still believed to be secure).
Presuming there’s some validation that SHA-1 names are unique, then that should be safe — the only way I can see one could do a pre-image attack is either fetching from a SHA-1 server (because then you don’t get the SHA-256 object name), which requires a second pre-image attack on SHA-1 (known to be feasible); or by having a second pre-image attack against both SHA-1 and SHA-256 simultaneously (and SHA-256 is still believed to be secure).