Figured out how to do this, if anyone comes looking for a solution. The answer is to build a shortcut that uses a URI to add the note directly to the mobile app. Inspired by the second shortcut at https://www.reddit.com/r/ObsidianMD/comments/owfz60/2_ios_shortcuts_for_obsidian_mobile/, I did this:
Trigger this script with Siri, then dictate the title and note body, and it’ll open the new note in the Obsidian app (with the title selected, for some reason… slightly annoying).