There isn't any code yet for loses or gains initiative at this point. The main issue is RM doesn't really define what that means so every GM will interpret it differently. I would love to hear a few perspectives on how it is used in people's games though so I can see if there is anyway I can provide some options and add it as automation in the ruleset.
You can try using this in your critical table definition and it should drag over the text so you can keep track of it:
<Effect type="string">Loses initiative</Effect>
I haven't tested it but should show up in the combat tracker as an effect. Then you would just need to adjust the duration if it is for more than one round.