It does take a bit of work, but I have successfully taken the bmp of a custom map, shrunk it, changed it to b/w, then touched it up and converted to a rle with the above tool. It does work as advertized.
I used the above tool to convert the osi rle to a bmp, then taken some of the "hand drawn" parts (like the town and the mountians) and copied it to the custom map. Since you've invested the time into creting a map, it's really not that much extra to do a little work on this.
You can see the results here:
The custom map: (partial area)
The rle:
-Ryandor