I have done this with mine, and it does seem to work. I have an old 50mm F1.7 from my ancient ME Super, and it works well. I think you can also set it to auto, and then it will use what ever aperture you set manually, but you might want to check that as it is a while since I have tried it.
Shot predominatly pentax since mid 70's.When switch to digital was made 6 years ago,having enough K,M,and A series glass made transition to pentax DSLR requesite.Pentax maintaining backward compatability means a lot to me,amazing statement of good faith on their part.Been some hiccups recently along corporate lines.With announcement of new bodies still having K mount and green button compatability,believe commitment to customer base reaffirmed.Love shooting old glass,insects come out shortly I'll slap on K200/2.5 with extension tubes, just get lost in the "itsy bitsy" stuff.Cant help but be more than interested in pentax return to "big glass class" and annoucement of DA 560mm F/5.6 super telephoto lens.
It's a a common misconception that old lenses wont stop down on modern DSLRs, but the stop down mechanism is still there, even modern lenses stop down via a mechanical linkage from the camera body when the shutter fires.
The so called "crippled" mount on modern bodies are missing a second mechanical link to the lens which used to register what aperture the lens was set to, this was done away with to reduce production costs and of course it was largely no longer needed as anything from a KA lens onwards would transfer aperture settings to the body electronically. Hence the reason that manual lenses need the camera to take a meter reading before exposure by manually stopping down the lens via the green button.
If you own or can get your hands on an older film body take a look inside the mount, on the left there is the stop down lever and on the top right is another lever which would penetrate into the lens and register the aperture setting.