I’m aware that the custom meta-data I add in the Win 3 beta will not show up in the inspector on my IOS version. The question I have is that if I
create custom meta-data (in my case, a date-formatted timeline) using the Windows beta,
then sync it to dropbox,
then work on the file on IOS,
then re-sync throw dropbox
then continue working on the file in the Windows beta,
will I permanently lose chages in the custom meta-data field?
No, you shouldn’t–the fields and metadata aren’t deleted, they’re just not used on iOS. Technically they’re not touched at all by the iOS version, which only ever reads from the project file used on the desktop versions and copies the relevant portions to its own mobile binder file for writing changes to, and those changes are then later merged into the “main” file when reopened on the desktop.
Now, do please be sure you’re making backups during this, so that if you do run into a bug whereby metadata isn’t being preserved properly in the beta, you can restore it! And in that case, please report the bug so we can get it fixed. 
Thank you. So far so good!
Don.
I have been defining and editing Custom Metadata a lot in the beta and syncing back and forth to iOS up to a half-dozen times per day without problem. This seems to be working well.
Keith mentioned middle of last year that adding Custom Metadata display, yet alone, editing capabilities to the iOS version would be quite difficult, so there is no plan to do that.
There have been times when I wanted to add or edit some of my custom metadata fields when working on iOS and one little (crude) trick I do for that, is to create a template (not project or document templates, but just a file with my custom metadata fields in them) in the Research Folder and then if I know I want to edit / input a custom metadata field, I cut and paste my text template into the Notes of the Document I want to place the metadata and then edit it in Notes, and then next time I am on the Windows platform, I move the custom metadata from Document Notes into the actual Custom Metadata field. A bit crude, but it works pretty well and has helped me not to forget to add it later!