I saved my own undo stack, I can dump a client connection, and get their undo stack object id or something like that. I planted some bricks, and saved it to a file and the file has stuff like this:
//--- OBJECT WRITE BEGIN ---
new ScriptObject() {
class = "QueueSO";
val14 = "144325\tPLANT";
val33 = "144363\tPLANT";
val52 = "144401\tPLANT";
val71 = "144439\tPLANT";
val90 = "144778\tPLANT";
val109 = "144816\tPLANT";
val128 = "144854\tPLANT";
That goes up to 500 so I'm assuming that you can undo bricks up to 500. Now, I know where to access their planted bricks, I might be able to do this, but this is going to have to be a client sided mod.