As you noted, a Participant license entitles a user to Read-Only repository access. Directly importing files through any repository interface (drag-and-drop in Web Client or otherwise) is fundamentally a Write operation. It requires the Import Feature Right which users with Read-Only access cannot use.
In addition to Read-Only repository access, users with Participant licenses are entitled to submit Laserfiche Forms. Forms can have file upload fields, and Forms processes can use Service Tasks to save those uploaded file attachments to a repository. That functionality may address your use case. If some Participant users need to be able to upload files directly to the repository through the Windows or Web Clients, I would recommend speaking with your Solution Provider about upgrading them to Full user licenses which provide that capability.
Hope that answers your question.