Wow, I'd love a 200 f2 lens, alas I won't ever have one without changing systems and I've no plans to do that.
I really like what the Nikon version has created here:
I guess the photographer was a fair way off to get the subjects in the middle of the bokeh and without that 200mm FL it's going to be hard to achieve this look.
I do think that if you plan on using a Pentax camera and really want to blur the background that you consider one of the ZF2 135mm f2 lenses with a Leitax adaptor to K mount.
It has the ability to isolate subjects with an OOF background without the need to be so far away from your subjects.
Of course the distance between the photographer, the subjects, the background, the FL and aperture all play a part in what you're looking for but if you're okay with manual focus this and similar lenses could be a good option to consider.
Tas
NOTE: the above images are not mine, attribution details are available by clicking on each photo.