Hi Darren,
I believe the labels of Journey Designer are hard-coded and the designer itself is not customizable too.
(In form editor, I can know that Journey Designer is called CJHostControl, however, after exploring through customizations of both Settings and PowerApps editor, I found that the control is not available in custom controls list, therefore it is not supported to be exported and customized.)
As workaround, you could show a notification to users when the customer journey is in draft status, to inform them that segment refers to the renamed entity.
(Execute the following script at OnLoad event of Information form of Customer Journey.)
function addNotification(executionContext) {
var formContext = executionContext.getFormContext();
var statusReason = formContext.getAttribute("statuscode").getValue();
// Add a notification to ribbon if record is draft status
if (statusReason === 192350000) {
var notification =
{
type: 2,
level: 4,
message: "Segment refers to Division, segment is the default name, but we renamed it due to business requirement."
}
Xrm.App.addGlobalNotification(notification).then(
function success(result) {
console.log(result);
window.setTimeout(function () {
Xrm.App.clearGlobalNotification(result);
}, 30000);
},
function (error) {
console.log(error.message);
}
)
}
}

( However, if we saved and published customization, the "Look for xxx" in the third lookup field should change.)
Regards,
Clofly