The problem is that if you mark a block as is-page:: yes when a parent block already is a page, you would have two Markdown files sharing a portion of the content. Then when you edit one of them from another tool, how could Logseq handle two conflicting files? It sounds impossible to me.
As an alternative approach, check: Option to treat specific blocks as pages. Or even better Alias property for blocks.
Another thing that would be nice is Logseq being able to mount some virtual read-only Markdown files. For example the results of a query. There are different ways to create virtual drives on desktop OSes, some more native but a cross-platform approach could be a local WebDAV server. It would be similar to the current HTTP API server but for files.