I know this isn't what you want to hear, but I don't know if it's reasonable for you to expect them to make sure every new feature they update on the new models is backward compatible AND they provide custom upgrade kits for all previous models? Not trying to be a jerk, but think about it. Imho, Harley is much better than alot of firms with making upgrade paths for many items for older models.
Alot of products out there are pretty rigid snap shots in time, especially in electronics, which are pretty much obsolete the second they hit the store shelves. I bought an entire high dollar (for me) home theater system 2 years ago, and it's ancient already and there's no possible way to update anything. You can't buy an upgrade kit to get from from HDMI 1.3a to 1.4? And you certainly can't upgrade a Std Flat screen to 3D. You live with it as long as you can, then you start over and buy a new one. I guaranty a BMW will have less flexibility in updating than a Harley by a long shot, so to counter your ascertion, I think Harley is pretty good in this area, and much better than most.
That all said, I installed the entire 660 and dash mount kit on my 2006 SEUC based on some of the threads posted here a few years ago when the feature first was incroduced in the new models. Many of us grabbed the parts books and figured out the restricted PN's and sorted it out. We tried a few things, fabbed a bit, and evenutally with a few tweaks here and there, we all got it done. I recall Spike and a few others were real trailblazers who helped alot of us - with zero MOCO help, support, or guidance. I was ordering stuff the dealers had no clue what it was for. With the right parts guy, and a correct vin for the needed part, it's not too hard to get exactly what you need, if you take a little time to research things.
While I realize it's slick in running the voice throught the stereo, muting the existing programming, with the NIM, etc, the fact is, just like the CB cutting in and out, it gets old quick. I always just mute it anyway, pay attention to the GPS, and leave the stereo cranked.
Again, not trying to be a jerk, but if you think about the added expense of going the full NIM route vs just doing the dash mount alone with the harness, you can get 95% of the finished product for a tiny fraction of the cost of going full NIM. I mean half the stuff the retarded Garmin voice blurts out you can't even understand anyway? If you absolutely cannot live without the Garmin computer chick guiding you, it would be much cheaper just to have Iron Cross install a rear input on the HU and just plug in the Garmin audio out from the harness right into the radio. Granted, it won't get you the auto-cut in and out, but in real congested areas, with a lot of turns/guidance, you can dedicate the input to the Garmin and hear everything through the radio. That'll save you a ton of dough. Then when you are in less congested areas, change the input back to the programming of your choice and motor on, leaving the Garmin in virtual mute mode. That's 99% there.
I hope this helps?