You can now establish backlinks with using the Markdown standard link syntax.
Use the new variable md-roam-use-markdown-file-links to control this function.
It is an opt-in customising option, and default is nil. Set it to t if you would like to use the Markdown link syntax for backlinks. This does not change the [[wiki-link]] behaviour, so you can continue to use it as is.
Currently, it does not have it's own insert function -- something to consider as a future improvement.
As Org-roam's backlink buffer (the side panel where backlinks are displayed) uses Org Mode, the Markdown links are displayed as raw text. This UI may have room for improvement. See how a backlink appear in the side panel in the screen shot above.
Despite its appearance, you can click on it to navigate to the linked file from the side panel (Org-roam's behaviour).