Can I do this with FrameMaker ExtendScripting?

Note also that FrameMaker markers have an internal limit of 255 characters. I am not sure if this has changed in later versions, but you should test this before you get too far into development.

There are a bunch of events triggered by FrameMaker, but I don't know if there is one that is specifically triggered when a marker is selected. There are a couple of general purpose events like:

Constants.FA_Note_BackToUser

Constants.FA_Note_PreFunction

Constants.FA_Note_PostFunction

Here is a general shell for setting up notifications. In this example, the script is looking for a paste event to happen, which is 803. I figured out the 803 by setting up the event and monitoring the iparam value as I copied/pasted text and objects.

function Notify (note, object, sparam, iparam) {
    
    var doc;
    
    // Handle the event triggered after a function is performed.
    switch (note) {
     
    case Constants.FA_Note_PostFunction:
        // Look for paste function.
        if (iparam === 803) {
            doc = object;
            if (doc.FirstSelectedGraphicInDoc.ObjectValid()) {
                // If the document has a selection, turn off its runaround property.
                RN.turnOffRunaround (doc.FirstSelectedGraphicInDoc);
            }
        }
        break;
    }
}
// Set the notification for after a function is executed.
Notification (Constants.FA_Note_PostFunction, true);