It’s somewhat quirky, but you can customize the osc messages sent by Reaper. In the dialog where you configure the osc ports for Reaper, there is also the possibility to select a template file and, if I remember correctly, also a button to open the folder. In that folder are several templates. You can copy the default (make sure to keep the extension) and edit (with a plain text editor) the copy you made. In Reaper select you file to be the template.
It will take some time (and an osc sniffer will come in handy), but it is possible.