Driving the outputs of a buffer
Backdriving is dicey business -- taking the buffer out's a better option

I'd desolder the buffer if I were in your shoes -- it's the only guaranteed safe way to do this.  If desoldering the buffer is utterly impractical (such as a HC125 in an underfilled BGA somehow) and backdriving it is your only option, here's the rules you need to follow, courtesy of TI:


  
  Backdriving should be used only when the state required at the node point in question can be reached in no other way
  The maximum permissible power dissipation of a device should not, under any circumstances, be exceeded.
  Outputs that are at a low-level state, as a result of their logic functions, can be raised to a level of Vo = 3 V for a
  short period by backdriving. The energy, which as a result of this backdriving is injected into the device
  (Vo × Iol × tpd), must not exceed 25 mW/s. The current that results in an output should not exceed a value of
  Iol = 300 mA. The pulse duration must not exceed td = 100 ms. To keep the thermal stress within acceptable limits,
  the duty cycle of the pulses (duration of the pulse ÷ duration of the period) should be less than 1:10.
  Outputs that are in a high-level state as a result of their logic functions can be lowered to a level of 0 V for a short
  time by means of backdriving. One output of a device can be short circuited to ground, in such a case, for maximum
  td = 100 ms. The product of the output current, the supply voltage, and the pulse duration (Ioh × Vcc × tpd) must
  not exceed 25 mW/s. If n outputs are simultaneously short circuited to ground, limit the total energy injected into
  the device under test (Ioh × Vcc × tpd) to 25 mW/s. To keep the thermal stress within reasonable limits, the duty
  cycle of the short circuit (short-circuit duration ÷ repetition time) should be less than 1:10.
  All voltages, including peak voltages of overshoots/undershoots, must be within the absolute maximum ratings on data sheets.
  Simultaneous backdriving of several outputs in parallel (wired OR) with a common current source is not
  permissible. Since current sharing cannot be predicted, there is danger of overloading the circuit.
  The chip temperature of the circuit under test must not exceed 125°C.
  Open-circuit (unterminated) lines should be avoided to prevent faults caused by reflection.