Listing 1
Making use of the spare upper bits.

   ;(this first ORI could be left out, but is included for clarity)
   ori  SIXBITS, MYRET1  ;tell routine to return HERE
   rjmp  BIG_MATH
FAKE_RET1:
   ...
   ori  SIXBITS, MYRET2\
   rjmp  BIG_MATH
FAKE_RET2:
   ...
   ori  SIXBITS, MYRET3
   rjmp  BIG_MATH
FAKE_RET3:
   ...
   ori  SIXBITS, MYRET4
   rjmp  BIG_MATH
FAKE_RET4:
   ...
BIG_MATH:  ;too big to be in-line multiple times
   ...
   addi  SIXBITS, 0x40  ;see if it rolls over from 0xC0 to 0x00
   brcc  FAKE_RET4  ;remember, with ADDI the C flag is reversed!
   addi  SIXBITS, 0x40  ;bump again until it rolls over
   brcc  FAKE_RET3
   addi  SIXBITS, 0x40
   brcc  FAKE_RET2
   addi  SIXBITS, 0x40  ;make sure we ALWAYS leave w/ upper bits = 0
   rjmp  FAKE_RET1<BR>