In SourceTree's Preferences, the Mercurial pane mentions `record` among the extensions. While not enabled by default, I can tick it.
However, there doesn't appear to be any GUI-level support for making use of this: given a file with multiple changed hunks of which I only wish to commit a select few, I wouldn't know how to select those. The only per-hunk buttons available are "Discard Hunk" and "Discard Selected Lines", which I assume will revert those changes, rather than skip them for this commit.