Selected, along with Shuffle, is now a valid <PlayItem> and follows the rules documented there for playitems:
Some options in the table below accept a parameter <playitems>, which is a list of one or more items, each item separated by a | (vertical bar, or pipe). Through the text, each item in <playitems> can be any of the following:
A filename: Example: C:\song1.mp3
A URL: Example: http:\\www.somewebsite.com\station.pls
A location within the Organization Tree: Example: TREEPATH=Playlists\Playlist1
Note: The TREEPATH=<location>\<item> specification is case-sensitive for <location> and <item> and the hierarchy separator is \ (backslash).
A device under CD & DVD and Handhelds: Example: DEVICE=G:
Therefore, you can do:
mc27.exe /Play selected - Plays any files current selected in the currently active View, appending them to the current list if it already contains items.
mc27.exe /PlayNext "selected|shuffle" - Adds as next to play any files selected in the currently active View, shuffling the new items first.
mc27.exe /PlayReplace "c:\some\path\to\song1.mp3|TREEPATH=Playlists\Playlist1|selected|shuffle" - Will clear Playing Now and replace the contents with the shuffled results of the currently selected files, the contents of the Playlist at Playlists\Playlist1, and the file c:\some\path\to\song1.mp3
Note: If any of the PlayItems contain spaces, or if you use the pipe character to separate multiple items, you have to wrap the playitems in double-quotes or the command interpreter will get confused and try to actually pipe the output (or consider them multiple parameters at the space).