The Zoneedit control panel/interface is not developed to allow forward PTR records. However, since BIND allows this record, there is a workaround which requires you to manually edit your zone file, then import it back in. The downfall of this, is you won't see those records in your DNS Settings page and can only change them by doing the export/edit/import again.
Load (manage) the domain in question in your Zoneedit account and click the Tools Tab.
Click the Bind Format link
Copy all contents in the yellow box and paste it into a TXT editor.
Add the PTR entries anywhere after the nameservers.
_sub-domain._tcp IN PTR target.hostname.com. (make sure that trailing period is there in the target hostname)
Once you have all the records added in, save the file and go back to the Tools tab and click the "Import" link. You should be good from there.
Once the zone is imported, you should be able to query something such as: