Not sure, how much effort that would be. But at the moment, neither Markdown LSP nor Helix itself seems to support auto completion of citation keys when writing papers/notes in Markdown.

Thus, this would be a great feature.

E.g. if the YAML header of a md/qmd file contains the line bibliography: /path/to/bibfile.bib, to be able to trigger auto completion of the included bibentries when typing @... with previewing the related entry of the bibfile.