Wow, set the way-back machine on this one! I used to program in assembly on the '02 family for years back in the 70's and 80's on the Atari VCS, computer family and coin-op arcade games. Ok, as I recall all the BEQ (branch on equal) instruction does is check the status of the 'zero flag' and branch if it is set. The zero flag gets set/cleared based on comparison and math functions.
Since you are asking about +/- numbers on the '02, you probably want BPL (branch if plus) or BMI (branch if minus) which are based on the sign 'N' flag which is essentially just bit 7 of the accumulator. So decrementing 0 would get you $ff and set the minus 'N' flag. You would interpret this $ff as -1, meaning 2's complement.
Update: Yes, I was perhaps a little round-about in my answer.. So, yes it is 2's complement, and the offset is as of the address of the next opcode that will be fetched, meaning -2 or $fe would get you back to your 'BEQ'. So $F0 $A7 would in fact branch backwards 89 bytes from the address 2 bytes ahead of the address of your BEQ