Here is a partial DASM file for 'outzone' in which you can see our routine :

013ABE: 4E71                       nop                           ; Ignore checksum error (ROM EVEN)

013AD4: 6022                       bra     $13af8                ; Ignore checksum error (ROM ODD)

000970: 4EF9 0003 FFE0             jmp     $3ffd0.l              ; Jump to our patched routine
000976: 4E71                       nop
000978: 4E71                       nop
00097a: 4E71                       nop

03FFD0: 48E7 FFFE                  movem.l D0-D7/A0-A6, -(A7)    ; Our patched routine starts here
03FFD4: 7005                       moveq   #$5, D0               ; Starting level - 1 (the value in D0 must be between 0 and 6)
03FFD6: D179 0024 0c02             add.w   D0, $240c02.l
03FFDC: 4CDF 7FFF                  movem.l (A7)+, D0-D7/A0-A6
03FFE0: 4A79 0024 0B4A             tst.w   $240b4a.l             ; Copy of the original code
03FFE6: 6704                       beq     $3ffec
03FFE8: 32BC 0004                  move.w  #$4, (A1)
03FFEC: 4EF9 0000 097c             jmp     $97c.l                ; Back to the original routine


To get this, copy/paste this in your outzone.xml file :

  <cheat desc="Select First Level">
    <parameter min="1" max="7" step="1"/>
    <script state="run">
      <action>maincpu.mw@013abe=4e71</action> <comment>Ignore checksum error (ROM EVEN)</comment>
      <action>maincpu.mw@013ad4=6022</action> <comment>Ignore checksum error (ROM ODD)</comment>
      <action>maincpu.mw@000970=4ef9</action> <comment>Jump to our patched routine</comment>
      <action>maincpu.mw@000972=0003</action>
      <action>maincpu.mw@000974=ffd0</action>
      <action>maincpu.mw@000976=4e71</action>
      <action>maincpu.mw@000978=4e71</action>
      <action>maincpu.mw@00097a=4e71</action>
      <action>maincpu.mw@03ffd0=48e7</action> <comment>Our patched routine starts here</comment>
      <action>maincpu.mw@03ffd2=fffe</action>
      <action>maincpu.mb@03ffd4=70</action>
      <action>maincpu.mb@03ffd5=(param-1)</action>
      <action>maincpu.mw@03ffd6=d179</action>
      <action>maincpu.mw@03ffd8=0024</action>
      <action>maincpu.mw@03ffda=0c02</action>
      <action>maincpu.mw@03ffdc=4cdf</action>
      <action>maincpu.mw@03ffde=7fff</action>
      <action>maincpu.mw@03ffe0=4a79</action> <comment>Copy of the original code</comment>
      <action>maincpu.mw@03ffe2=0024</action>
      <action>maincpu.mw@03ffe4=0b4a</action>
      <action>maincpu.mw@03ffe6=6704</action>
      <action>maincpu.mw@03ffe8=32bc</action>
      <action>maincpu.mw@03ffea=0004</action>
      <action>maincpu.mw@03ffec=4ef9</action> <comment>Back to the original routine</comment>
      <action>maincpu.mw@03ffee=0000</action>
      <action>maincpu.mw@03fff0=097c</action>
    </script>
  </cheat>


Select your starting level with the cheats (use the left and right arrows), reset MAME (F3) to be sure, enter the debugger, then save range 0x000000-0x03ffff to a binary file ...

I now let you code a tool that splits this file into 2 smaller ones (one for EVEN, one for ODD), then find a way to "burn" these 2 smaller files into ROMS which you can then plug on your PCB ...