It is - as far as I know - not possible to open documents programmatically in editorial (the OpenDocument workflow node only works for constants). However I did tinker a bit around which made me came up with this little workflow [1]. It will try to preview a markdown file from a relative file path in a markdown link as a HTML preview. The workflow will select the link closest to your current cursor position. The workflow is also merely more than a concept, you will have to flesh it out on your own.
Here [2] is an example document to test it on (you will also have to create a file file_b.md in the same folder, a folder moo, and a file file_c.md in this folder moo to make the links work. If you put your cursor on one of the links it will open a preview window with the rendered content of that file.
Cheers
zipit
[1] http://www.editorial-workflows.com/workflow/5870624836681728/DraBnh7o3RY
[2] https://pastebin.com/UGzsSqMN