Feature summary: Special:SuggestedInvestigations has a "notes" column where one can comment. The content should be parsed as wikitext.
Benefits: Ease wikilinking to users, SPI cases, unblock requests, etc. There already exist examples with brackets to indicate links on enwiki.
I believe this can be implemented through CommentFormatter.php, which is what the 'reason' in the CU log also uses.