I know what your talking about but do you have a serial command for RUN ? (the equivilent of clicking the IDE RUN button )
My point is that when you program an ATtiny85 with an UNO as ISP there are several steps, the first of which is loading the ArduinoISP sketch, followed by changing the board type (do you have a serial command to do that ?), followed by opening the ATtiny85 sketch you want to load and last but not least clicking the LOAD (RUN) button in the IDE. Are there serial commands to do all of these steps ?
This is probably all too theoretical to bother with but I will make one more comment …
The things you mention (RUN and changing the board type, opening the Attiny sketch) all happen in the IDE which is on the PC.
In the normal scheme of things the IDE sends a sketch to an Arduino over whatever serial connection you choose. So why wouldn’t it send over bluetooth to the Arduino that has the ISP loaded onto it?
I’m not saying it WILL work because I haven’t tried it. I just think your objections are as theoretical as my idea.