I've also created a reboot icon, however, I didn't have to move it since it showed up nicely above the exit and Suspend menu entries.
As I understand the documentation of matchbox, it is not possible to determine the order of program (or action) icons using the .order files.
So there must be another way to determine the order, or at least, to influence it.
Now I tried to modify values inside the reboot.desktop file in order to change its position, but to no avail.
I think I have created it by copying the rotate.desktop file and modifying the copy accordingly. The reboot item shows up directly under the rotate icon. Maybe this is the key. However I don't know what should cause this. File creation date?
In order to test this, I created a new .desktop file with almost the same contents as the reboot.desktop file.
It shows up as the seecond from the bottom.
I then renamed that file from test.desktop to 000test.desktop and it changed its position with Exit X.
So the file name seems to have an effect.
So much for now. Sorry I cannot help more.
in my case, the last created item is always the one at the bottom, so i created a new suspend item and deleted the old one