This is related to this question, but it is more a special case of it.
I reviewed a manuscript, which was submitted to a reputable traditional journal. The journal has a single-blind review system. I liked the manuscript a lot, as it was closely related to my research interests. However, the authors did not do a great job in reporting the manuscript, and I provided a long review with several suggestions for improving the report.
I suggested major revisions, but the editor rejected the paper. That is ok for me; it is the editor's responsibility to take such decisions. What is important is that the process is now closed.
By re-reading my review together with the other two referee's reviews, I came up with a couple other suggestions that I truly believe would benefit to the manuscript. Would it be a bad practice to contact the authors now that the paper is no longer under review? I do not mind unveiling my identity, and I would do that only because I want to advance the research in the paper. That is, I just want to help them.