By the way, I found a fix for this.

First of all, an explanation; in the export_smd file's "writeSMD" function, this is the code used for writing bone positions and angles.


The offending functions here are mostly parentMat.inverted(); inverting matrices is done in C, and according to my own tests, some precision is lost in the result.
Finding the bone's matrices is actually completely unnecessary, as Blender already provides a pose bone's correct position/angle on the current animation frame with the bone.location/bone.rotation_euler variable (these variables are what are displayed when you're editing a bone in the Blender menu.) Upon switching out this code with using those variables instead, in my tests everything exports perfectly with no precision errors; positions and angles alike of bones are exactly like in my original .smd.

Upon importing an .smd animation and exporting it in the same format (with no changes made whatsoever to it, mind you), I noticed that floating-point precision was lost for some of the bone positions (I don't believe any was lost for the radian angles, peculiarly) in its frames.

Here are some examples:

Original SMD, frame 50: https://pastebin.com/yX6k5gS3
Re-exported SMD, frame 50: https://pastebin.com/NmYkN25Z

Here are some examples of the loss of precision.

Bone #17, original SMD, frame 50:
17 3.698241 0.000000 -0.000000 -0.003276 0.002285 0.091873

Bone #15, original SMD, frame 50:
15 0.000000 4.043968 -0.017929 1.577360 -0.131264 1.576111


Bone #17, reexported SMD, frame 50:
17 3.698244 0.000000 -0.000000 -0.003276 0.002285 0.091873

Bone #15, reexported SMD, frame 50:
15 0.000000 4.043965 -0.017929 1.577360 -0.131264 1.576111

These differences, while obviously minor, are still there, and seem fixable.