Nerdiest byte sequence that I ever thought up was: EB 03 C3 yy xx
If you create a .COM file with those 5 bytes as the first ones, and look at the disassembly (I'm assuming an x86 disassembler here..), you'll see 'JMP SHORT 3', followed by 3 garbage bytes. Nothing ususual here...
If you look at a Z80 disassembly of those same bytes, that translates to 'EX DE,HL; INC BC;'. Exchange the content of 2 registers, increment another one, nothing special. The 3rd byte is 'JUMP' followed by the 16-bit address specified as yy xx.
So, if you create a .COM file with the 5 bytes as above, followed by 8088 code, followed by Z80 code (at offset xxyy + 0x100 in the source), you'll have a .COM file that runs on MS-DOS and (i.e.) CP/M...