nwellnhof was close, but to get the literal SQL to SQL::Abstract, I had to do a coderef like so:
__PACKAGE__->has_one(
'other_thing',
'MySchema::OtherThing',
sub {
my $args = shift;
return {
qq{$args->{'foreign_alias'}.id} => { q{=} => \qq{CAST($args->{'self_alias'}.dept AS int)} },
};
},
);