Jeremy,
I own both GL690 and GM670 bodies. The GM670 has a VF'er of higher maginfication than the GL690, and I find it is well suited for use withe the Fuji 150/5.6 I own. Furthermore no external VF'ers are required, and parallex compensating framelines for 150mm are shown with the 100mm frames in both bodies at all times, except when close focusing, meaning I think under two meters the 150mm frames disappear. Understand that the 100mm can close focus to 1 meter.
Both my blackslides work reliably, and this is not often the case if you want to perform lens changing. My approach is to carry two Fuji's as I seldom change lenses in the street. If you want to enlarge your arms and thicken your neck where your neck gets kinda monsterous and disproportional like a footbal player I suggest carrying two rigged Fuji's is for you.
The 150/5.6 is a Sonar formula and in 6x7 is about 70mm FOV. In 6x9 the 150mm is about a 60mm (like a long normal). In this reguard paired with a 100/3.5 which has a 40mm FOV in 6x9 I can see great utility of shooting a 40-70 FOV Fugi long-short pair. Also a 40mm FOV and 60mm FOV is available with just a GL690 body and the 100/3.5 and 150/5.6.
I'm lucky in that I have a Fuji rear lens cap that allows me to carry my GL690 and two lenses if desired. The hood for the 100/3.5 effectively can be shared on my 150/5.6, and the 150/5.6 is only slightly longer than the 100/3.5. I find the 100/3.5, a Tessar formula, to be a particularly nice lens with ultra smooth bokeh when shot wide open and close focused. The 150mm really hits its stride not for close focusing, but with distant shots where compression of space is its strong point.
BTW the 150/5.6 is a very rare lens. As you know I'm a lucky guy. I do not find the 150 hard to focus, especially on the GM670 (higher magnification VF'er), but I could see having difficulties with the 180.
Dante's post about using a Pentax 67 for long lenses is a great suggestion. Understand that I also own a Pentax 67II with AE prism. I find the Fuji's are easier to load than my Pentax if that matters to you.
Cal