I've done it - actually I took all my teleconverters and close up lens attachments and stuck it all on the MPE in one go - it was nuts. It also let me take a photograph of the ball of a ballpoint pen and the nib section with the ball filled a good 3/4 of the frame or so.
You can most certainly use teleconverters, however if you've used the MPE 65mm at 5:1 you'll already know how hard it is; going higher in magnification makes it even harder still! By all means try it, but expect it to get really tricky and you might find it ends up a tripod only situation.
In fact when I did my "silly" experiment I wound up locking the camera down on the tripod and using the focusing rail under the subject to move it closer as that was easier (the lens was rather long once I'd added all the parts so with the weight and size it was easier to move the pen).