Hi @higuywhatsthestory,
No, there's no marker support (as in Logic markers/ MIDI marker meta events) supported in MusicXML. However, there are several elements in MusicXML that could be repurposed as markers:
The <rehearsal>, <words> and <credit-words> elements to name a few.
For example, Plain Text added in your score in Logic with the Text tool will translate to the <words> element when exporting your score as MusicXML.
Just give it a try and add some text with the text tool (Plaint Text or customized with Layout >Text Styles) and see what looks best in Sibelius. I was experimenting and ended up with <credit-words> elements after creating these in a test score:
I don't have Sibelius so don't know how that would look there...
J.