There is a tool that may fit into your requirement. It is called ZPL Label Designer Online | koditPaperFlex. This company, one of Zebra's strategic partners, has designed a Web-browser app to design labels. This app can be even embedded in other Web-Browser tools. The tool will allow you the creation of the zpl code to send to the printer, and others powerful features.
In the particular use case you mentioned, this tool would be able to run on iPad and generate the ZPL of the label there.
For more information about this tool, I will recommend to visit the link below.
For other alternatives on label designers please, visit our page of partners that also offer these tools, scroll down and go to the section of our Stretegic Partners, please, review the options for Label Design Software.