That's a myth that should have a quick and painful death.However painful you think it is, multiply it by 10X with this i-drive setup.
The iDrive has never been any more complex to add aftermarket amps/processors than any aftermarket headunit. Not the F30 iDrive (be the Professional Radio or the CIC Nav unit), not the E90, E60 or any other iDrive since 2002.
All iDrive units can either output analog high/low level balanced or digital audio signals. The easiest signal to work with aftermarket is analog (Stereo, HiFi), obviously. The digital signals (Logic7, Individual Audio, Harman Kardon, Premium systems), can be bypassed by either OEM amp analog outputs, replacing the OEM amp with a MOST converter, or simply by coding the unit to output analog. This can be done without cutting any OEM wires.
In the particular case of the OP, there is no OEM retrofit of the HK system. However, a Logic7 processor can be added to the F30 HiFi system by replacing the OEM amp with a JBL MS-8 directly. No OEM wires to cut, splice or tap, just unplug the OEM amp and then plug the MS-8 instead. The OEM speakers do not need to be replaced, and the F30 in particular has a center speaker from factory so the Logic7 processing will fit right in without any other modifications. This is 100% OEM reversible.
So yes, any aftermarket audio device can be integrated to the iDrive without permanent modifications if the installer knows what to do. It is that simple.