After some trial and error, I was able to make the following bash script, which will take the X clipboard selection (containing math operations you want to perform), run it through bc (to do the math), replace the X clipboard with the result, and paste it, overwriting the previous result:

    output=$(echo "$(xclip -o)" | bc) # take the X clipboard and run it through bc
    sleep 0.2s # wait before continuing or else this won't work
    xdotool key BackSpace # clear the selection
    printf $output | xclip -i # write the output to the X clipboard
    xdotool click 2 # paste the result by clicking the middle button

I set this as Super-E on my computer and probably will use it every once in a while for programs which don't implement math input.

In other words, with this you can select text containing mathematical operations, press a key combination, and the selected text will be replaced by the result of the math.

Improvements welcome. There probably are better ways to do this.