LA-2a's and 1176s have a problem with this sort of thing. You can't buffer and have a galvanically isolated output without building a fully-floating (completely isolated) power supply for the buffer.
The whole purpose of the LA-2a and the classic iterations of the 1176 having passive moving-coil meters is that they sit directly across the outputs and show loading effects. However since all the outputs are galvanically isolated transformer secondaries, the meter has to be able to operate without any connection or reference to the unit's power supply. That rules out a buffer unless you're willing to build yet another isolated supply in there, or you're willing to sacrifice the isolation.
Here's why you probably shouldn't worry about the buffer on these devices:
Since the distortion (typically about 0.3%) only happens when the meter is loading the output. Therefore, when the meter is switched to 'GR' mode, the distortion disappears... no need for a buffer. -Since most of the time, the compressors are operated displaying Gain Reduction, and the output is usually only checked occasionaly during setup, there's no real need to go to all that trouble. -Just switch to 'GR' when it's operating and the problem goes away!
Keith