Unfortunately, this means the entire set of progress updates looks to grep, ack and all other line-oriented tools as a single line. That's why they seem to "lock up" and print nothing until ffmpeg finishes.

Also unfortunately, I can't think of a similar highlighting tool that operates on anything but a standard Unix line basis. You could try something like:

frame=   85 fps=0.0 q=28.0 size=       0kB time=00:00:03.79 bitrate=   0.1kbits/s dup=1 drop=0 speed=7.55x    
frame=  129 fps=129 q=28.0 size=       0kB time=00:00:05.65 bitrate=   0.1kbits/s dup=1 drop=0 speed=5.64x    
frame=  172 fps=111 q=28.0 size=     256kB time=00:00:07.42 bitrate= 282.6kbits/s dup=1 drop=0 speed=4.81x    
frame=  213 fps=104 q=28.0 size=     512kB time=00:00:09.17 bitrate= 457.3kbits/s dup=1 drop=0 speed=4.49x    
frame=  254 fps= 99 q=28.0 size=     512kB time=00:00:10.85 bitrate= 386.3kbits/s dup=1 drop=0 speed=4.22x    
frame=  295 fps= 96 q=28.0 size=     768kB time=00:00:12.56 bitrate= 500.7kbits/s dup=1 drop=0 speed=4.08x    
frame=  333 fps= 93 q=28.0 size=    1024kB time=00:00:14.14 bitrate= 593.1kbits/s dup=1 drop=0 speed=3.94x    
frame=  382 fps= 93 q=28.0 size=    1024kB time=00:00:16.19 bitrate= 518.1kbits/s dup=1 drop=0 speed=3.93x    
frame=  428 fps= 93 q=28.0 size=    1280kB time=00:00:18.11 bitrate= 579.0kbits/s dup=1 drop=0 speed=3.92x    
frame=  473 fps= 92 q=28.0 size=    1536kB time=00:00:20.00 bitrate= 628.9kbits/s dup=1 drop=0 speed=3.91x    
frame=  519 fps= 92 q=28.0 size=    1536kB time=00:00:21.93 bitrate= 573.8kbits/s dup=1 drop=0 speed= 3.9x    
frame=  567 fps= 92 q=28.0 size=    1792kB time=00:00:23.91 bitrate= 613.9kbits/s dup=1 drop=0 speed=3.89x    
frame=  601 fps= 90 q=28.0 size=    2048kB time=00:00:25.32 bitrate= 662.6kbits/s dup=1 drop=0 speed= 3.8x    
frame=  637 fps= 89 q=28.0 size=    2304kB time=00:00:26.83 bitrate= 703.3kbits/s dup=1 drop=0 speed=3.75x    
frame=  684 fps= 89 q=28.0 size=    2304kB time=00:00:28.77 bitrate= 655.9kbits/s dup=1 drop=0 speed=3.74x    
frame=  720 fps= 84 q=-1.0 size=    2924kB time=00:00:30.01 bitrate= 798.0kbits/s dup=1 drop=0 speed= 3.5x    
...

Unfortunately, this means the entire set of progress updates looks to grep, ack and all other line-oriented tools as a single line. That's why they seem to "lock up" and print nothing until ffmpeg finishes, and I can't think of a similar highlighting tool that operates on anything but a standard Unix line basis. 

You could try something like:

frame=   85 fps=0.0 q=28.0 size=       0kB time=00:00:03.79 bitrate=   0.1kbits/s dup=1 drop=0 speed=7.55x    
frame=  129 fps=129 q=28.0 size=       0kB time=00:00:05.65 bitrate=   0.1kbits/s dup=1 drop=0 speed=5.64x    
frame=  172 fps=111 q=28.0 size=     256kB time=00:00:07.42 bitrate= 282.6kbits/s dup=1 drop=0 speed=4.81x    
frame=  213 fps=104 q=28.0 size=     512kB time=00:00:09.17 bitrate= 457.3kbits/s dup=1 drop=0 speed=4.49x    
frame=  254 fps= 99 q=28.0 size=     512kB time=00:00:10.85 bitrate= 386.3kbits/s dup=1 drop=0 speed=4.22x    
frame=  295 fps= 96 q=28.0 size=     768kB time=00:00:12.56 bitrate= 500.7kbits/s dup=1 drop=0 speed=4.08x    
frame=  333 fps= 93 q=28.0 size=    1024kB time=00:00:14.14 bitrate= 593.1kbits/s dup=1 drop=0 speed=3.94x    
frame=  382 fps= 93 q=28.0 size=    1024kB time=00:00:16.19 bitrate= 518.1kbits/s dup=1 drop=0 speed=3.93x    
frame=  428 fps= 93 q=28.0 size=    1280kB time=00:00:18.11 bitrate= 579.0kbits/s dup=1 drop=0 speed=3.92x    
frame=  473 fps= 92 q=28.0 size=    1536kB time=00:00:20.00 bitrate= 628.9kbits/s dup=1 drop=0 speed=3.91x    
frame=  519 fps= 92 q=28.0 size=    1536kB time=00:00:21.93 bitrate= 573.8kbits/s dup=1 drop=0 speed= 3.9x    
frame=  567 fps= 92 q=28.0 size=    1792kB time=00:00:23.91 bitrate= 613.9kbits/s dup=1 drop=0 speed=3.89x    
frame=  601 fps= 90 q=28.0 size=    2048kB time=00:00:25.32 bitrate= 662.6kbits/s dup=1 drop=0 speed= 3.8x    
frame=  637 fps= 89 q=28.0 size=    2304kB time=00:00:26.83 bitrate= 703.3kbits/s dup=1 drop=0 speed=3.75x    
frame=  684 fps= 89 q=28.0 size=    2304kB time=00:00:28.77 bitrate= 655.9kbits/s dup=1 drop=0 speed=3.74x    
frame=  720 fps= 84 q=-1.0 size=    2924kB time=00:00:30.01 bitrate= 798.0kbits/s dup=1 drop=0 speed= 3.5x    
...

You'd probably also want to disable stdio buffering between the components in this pipeline for maximum responsivness:

stdbuf -o0 ffmpeg ... 2>&1 | stdbuf -i0 -o0 tr \\015 \\012 | stdbuf -i0 -o0 grep ...