Passing a blitzcommand through a string

You'd have to parse the string and given the command you specify, have it call the appropriate actual Function/Method.

Select command
    Case "startgame"
        StartGame()
    Case "highscores"
        HighScores()
End Select

Or something similar... but either way, you'd need logic hard-coded somewhere.
Unlike with say, Java, which lets you, through reflection, create classes and call methods, access fields, etc... using strings.