Wow, that was fast. Thanks to @STerliakov for finding a related post!

The solution is generics. So the BusinessMan declaration becomes

class BusinessMan[BriefcaseType: Briefcase]:
    # snip

    @classmethod
    def from_briefcase(cls, briefcase: BriefcaseType) -> Self:
        # snip

which indicates that from_briefcase() takes a BriefcaseType, which is any subclass of Briefcase.

The declaration for BiggerBusinessMan becomes

class BiggerBusinessMan(BusinessMan[BiggerBriefcase]):
    # snip

    @classmethod
    def from_briefcase(cls, briefcase: BiggerBriefcase) -> Self:
        # snip

which says that BiggerBusinessMan inherits from the variant of BusinessMan where BriefcaseType is BiggerBriefcase.

Type annotations are sublime.

Pleasure doing business!