Well you have choices:
Use 'ljmp $SOMESEG, $mylabel + 0x100' which compensates by manually increasing the offset of 'mylabel'. The problem with this method is that you'd have to do this compensation every time you needed an absolute memory address (Eg A data reference/long jump).
Or
Use ORG 0x100 and make sure you don't use any absolute memory addresses before you have done a far jump to the copy. In my example this would mean the long jump goes to SOMESEG:0x0107 instead of SOMESEG:0x0007. This is effectively the same as the first option, but by changing the start address (ORG) I let the assembler add 0x100 to labels automatically, saving me the effort of doing it myself.
Or
Alter 'SOMESEG' so that the ORG 0x0 assumption is still true. In my example this would mean changing SOMESEG to 0x9010 ( 0x9000:0x0100 = 0x9010:0x0000 ). This would be the preferred option wherever possible.