I've never used a MII mobile, but in the stations (with only the regulator card in the shelf) I've tagged onto the mic audio line on the backplane.
Yes, I did need to use the URI amplifier.
I THINK that I had to bring the deviation pot way up. Enough that the exciter was effectively not doing any limiting. Relying on the software DSP.
I'm a bit hazy on that point though, it's been a while.
The PLL exciter was mentioned. On those you can inject composite (voice and PL) into the exciter CG input. I measured mine to be dead flat in response (no pre-emphasis at all)
to at least 8 KC. Which makes sense as from what I've been told, that's where they injected the EDACS data.
Yes, the microphone has an amplified cartridge. Hence MIC DC on the mic audio line. But the URI has a 10uf blocker cap on it's inputs and outputs if believe.