I'll answer my own question...
It appears there's either a bug in the firmware or the documentation is incorrect. Reading the file into an roByteArray and then using AsyncPostFromString with the roByteArray ToAsciiString() interface works - GetString() returns the response body. When using AsyncPostFromFile, GetString() does not return the response body.
Edit: I could have just used AsyncPostFromString(ReadAsciiFile("pkg:/command.txt")) and skipped the roByteArray.
-JT
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