When exporting to mp4, can I specify an area of the timeline for higher quality compression?

You could do it with ffmpeg but it will be a pretty manual process. I tested it out and it should work. Basically you'll be splicing together high and low quality encodings for different time frames.

For the setup you could approach it a couple ways, but for simplicity let's encode two versions with Premiere: One with the main target bitrate, and another with the higher bitrate. Meaning you'd encode the entire timeline for both versions into two separate files, then we'll extract specific time ranges from each, then combine them for a third final hybrid version.

First extract out the video parts you don't want to change like this, which you'd run on the normal quality version:

ffmpeg -y -i input.mp4 -ss 00:00:00 -to 00:00:01.500 -c copy output_part1.mp4

Where the -ss and -to parameters are starting and ending times, in "HH:MM:SS.milliseconds" format. To have it to go the end just leave out the "-to" part. So presumably you would do that command twice, one for part 1 (before the specially handeled part) and one for part 3, after it.

Then, you'd do the same thing as the command above, but doing it for the higher bitrate video and just for the segment that you want extra bitrate, which we can assume is part 2. 

Then you need to concatonate them together with a command like this:

ffmpeg -f concat -i list.txt -c copy output.mp4

Unfortunately I could not for the life of me figure out how to just put the list of files in the command line command, so you need to take the extra step of creating a list text file which will contain the list like this (don't forget the 'file' part at the beginning of each line)

file output_part1.mp4
file output_part2.mp4
file output_part3.mp4

Then that should work. I'll attach a test example (an animated emoji video about 3 seconds long), where the first half is around ~2000 kbps and the second half is extremely low (12kbps) to make the difference obvious.

Note: I would inspect the video because I've seen discussions about potentially having issues with keyframes considering the GOP encoding. It seems that might have been improved with recent versions of ffmpeg though not sure.