Well, with almost three pages of comments, I don't know how much I can add, but I felt like it anyway, mostly because I've spent most of my active photographic life shooting slides, and I've learned a few things about exposure.
I can understand you're not wanting to sideline your two most capable cameras with slide film. But my recommendation is that you do just that. Sideline one of them. You don't use both cameras in a photo shoot do you? If not, then load one of them up with slide film. I'm not familiar with the EOS A2, but I am familiar with the Nikon F3. And one of the things I like best about the F3, when it comes to shooting with slides, is it has a very tight metering configuration. Prior to all the multi-segment hooey, most Nikons used a 60/40 metering system. Within these Nikons' viewfinders, you'll see a largish circle. 60% of the metering weight occurs within this circle, and 40% of the weight occurs outside of the circle. This has been a tried and true system that has worked very well for many years. But because the F3 had an "A" auto-exposure mode, and because Nikon knew that this camera would be used in fast-paced pro environments, Nikon elected to tighten up their metering with the F3. It has an 80/20 system.
So, I felt comfortable using the F3 with its heavier center weighting. It made it easier to place the circle and move it around and evaluate exposure. But that was with me shooting the F3 in manual mode. I finally decided to suck it up, though, and give the F3 a try using its "A" setting and slide film. I can still recall that I took the camera to an air show for that first event. This worked out well because all I had to do was set the aperture and focus. I'll admit that when I got the slides back, I was amazed to find that not one single slide was off even a bit. They were all perfectly exposed. Well, this pretty much sold me on using the F3 with its "A" setting. Sure, I still shot in manual mode, but I felt comfortable when I set it to "A".
As for the other cameras you mention -- the Canon AE-1 and the Pentaxes -- these cameras all share one trait. They use a moderately centerweighted metering system. My first 35mm camera was an AE-1 and I have more than a littlle experience with shooting slide film with it. The biggest problem I had with it was believing the meter. It was much too sensitive to any bright light sources in the frame and woulld stop the lens way down for them. The result was my slides would come back severely underexposed. This was quite infuriating until I understood what was happening. Once I knew what to look out for, my number of keepers improved. But I still didn't like the idea of using a camera that would fool me that bad. So, I switched to an FTb, and later an original F-1, both of which use a selective area metering system, which uses a central rectangle that occupies only 12% of the image area. I got very used to using this rectangle with slide film and, since metering occured only within that rectangle, I could move it around the scene and get very accurate measurements for my exposures. It was an excellent system. Much better than that used on any of Canon's A-series cameras. Canon finally addressed this problem with the introduction of the T70 and its partial area metering option.
Now, I've spent all this time typing all the above because of the school of hard knocks I attended as a photographer who shot almost 100% slides. I got to where I was pretty good at it, too, and not by relying on any multi-segmented metering gizmos either. What I learned was that slide film can handle quite a bit of under exposure -- sometimes as much as two stops -- but it can't tolerate more than about 1/2 stop overexposure. Tops. Any more than that and your slides will have burn through in the white areas. So because of this I would try to expose for the highlights, if they weren't too severe, else I'd expose for a middle ground and let the highlights fall where they may. Sometimes this can result in some pretty dark shadows, in which case I liked to have a flash handy for fill-flash. Fill-flash with slides works great. I was most often using a Vivitar 285 back then, which is a non-dedicated flash with ranges of exposure that can be selected based on lens aperture and ISO. So I'd set it for the right range and I'd let the flash handle the duration. I came to trust its exposure instincts.
There's one exception to the above exposure methods I followed and that was sunsets. The first few times I went out with the goal of shooting sunsets, it bugged me that I wasn't able to get good sunset photos with slide film. I mean, I'd have a perfectly exposed sun, but then everything else would be black. So I got to thinking about it. If everything else was black, but I want to see this everything else with some amount of color in it, then obviously I need to increase exposure. But where do I meter? Well, I got to looking all around me at sunset one evening and I noticed that the sky and the clouds directly overhead were perfect. The sky was a deep blue and the clouds were pinkish from being illuminated by the setting sun. So it dawned on me: meter directly overhead. So that's what I did, and that group of sunset photos I took were some of the best exposed sunset photos I've ever taken. Now, there is a downside to this technique and that is, the sun goes white. But so what, I thought. Everything else had just the colors I wanted, including the sky immediately around the sun. So I figued I could live with it being white. And I've done so ever since.
