A common impedance for instrument amps is 47k
Your guitar will impose its waveform on the mic line with the same ease the mic did!, there will be a resistor from + battery to the mic connection, your multimeter input resistance is so high the resistor will have no effect and you'll read battery voltage.
You just need the capacitor to prevent dc from reaching the guitar, the pickup in the guitar allthough it'd probably survive would sound totally different without it probably really bad as its core would saturate with the dc.