a "Create event for a shared mailbox calendar" action
an "Update event for a shared mailbox calendar" action
a "When a new event is added to a shared mailbox calendar" trigger
Where these actions are permitted if you are a member of the shared mailbox.
This would allow the creation of a Company Travel shared mailbox where the travel is shown on the calendar, and emails gernerated by the approval flow are in the Sent folder for the mailbox. The sequeunce would be: An employee adds an event to the calendar, which triggers an approval process, which results in a reject email or a updated event with Approved in the body.