I know plenty of others have done this to boxes so hopefully someone can chime in with their experience.
Absent that, I can tell you that mics have different amplification and impedance needs, so aren't really a good fit. They usually have their own input circuit.
You could try it, but may not sound good.
Since you have no external input preamp (no jacks!) you could tap into the tape preamp like some cheaper boxes do.
Just poking around, I noticed that the Lasonic TRC-935 does this, you can see the schematic at the end of its owner's manual (on Boomboxery library).
It's a phono input, so the resistor values are wrong, and the schematic is kind of hard to navigate, but you get the idea. Another schematic that uses this method (and has resistors meant for a line-in signal) may be better.
