Unfortunately I am not familiar with OMP, but with the remote galleys functionality in the stable OJS release, thus maybe I could help somehow.
I will write the necessary steps (having OJS in mind) and try to comment/adopt them for OMP:
1. Integrate the option into the editor interface:
In OMP this could be done in "Add publication format": I think the current publication format drop-down list shouldn't be changed, because also if remote, it can be audio, digital, hardback or softback, right? Thus I would just add a new input text field for remote URL + maybe a description that if the remote URL exists the reader would be redirected to the remote URL.
2. This should be considered in the database model:
A new column 'remote_url' should be added to the database table 'publication_formats' -- the new field/column 'remote_url' should be added in the 'omp_schema.xml'.
The get/setRemoteURL() should be added to the class classes/publicationFormat/PublicationFormat. These functions and the new column 'remote_url' should be considered in classes/publicationFormat/PublicationFormatDAO.
3. The form controllers/grid/catalogEntry/form/PublicationFormatForm should be adopted.
4. And finally it should be considered for the readers:
Here I am not sure what would be best way to do it. I think the remote publication format would be automatically displayed with all other formats under the cover image. I am not sure if the remote URL should be displayed here or maybe in the download area. However, the handler pages/catalog/CatalogBookHandler should be adopted as well as the appropriate templates in templates/catalog/book/.
I hope I could help a little bit, so that you know where and/or how to start with this?