Impedance of microcontroller pins with no power supplied?
The impedance of microcontroller pins is likely quite high.  However, the issue is that these pins likely (I'm not familiar with that micro and haven't looked it up) have protection diodes or similar circuitry to Vss and Vdd.  The net result is that they look like high impedance within one voltage drop of ground (and power since power = ground when unpowered).  If you try to drive it higher than that, it may take significant current and it may also damage the micro.  In some cases it can even sortof power up the micro, but not how that is intended.

There are generally two ways to deal with this

Power up both units during the manufacturing test and programming process.  Then you have normal data lines that behave as they are supposed to.  Make sure the micro sets these to high impedance.  You may have to load different code in it temporarily for that purpose.  It's not unusual to have special production test code, or modes for production test in the operational code.

Put enough resistance in series so that the micro won't get hurt (good luck finding clear specs on that though), and won't load the signal to the point the programming jig can't handle.