I use a deglassed -a 50 f/1.7, I had several of them lying around so didn't feel too awful about butchering it.
Just for the record, my rear sync issue was a very very specific one, so I hope I'm not prodding people down the wrong path. I use manual mode for some of my
extreme macros because I mix ambient and flash. I use things like objectives and reversed enlarger lenses which Pentax classifies as 'manual' because they're not modern lenses with aperture contacts etc. On K-7, the Metz 58-II worked fine using it on its manual mode (ie the one with 1/2, 1/4, 1/8 etc) settings with rear sync also set, used as master to trigger a bunch of Pentax 540s and 360s. This works just fine (which is why I was surprised to hear this being a problem on 645d, they came out the same time I thought?).
But, something has changed on the Pentax end with the K-5. On the K-5, using exactly the same lens and setup above, the rear sync flash comes after the curtain is already down, which is stellarly useless. To make it come where it's supposed to, it needs to have a modern lens mounted (ie not a manual one in Pentax speak like I use, objectives, reversed etc). So, to fool the K-5 into thinking it has a modern non manual lens, I take a lens with contacts (-a 50 f/1.7) but remove all optics, and using araldite and bits and pieces just basically make an extension tube out of it.
Fwiw, I wanted to use this new 40mm 9mm lens for this purpose but the aperture didn't look large enough to me for a decent image circle, hence the use of the 50 which has a huge image circle with its 1.7 aperture.
Hope that helps!