I would expect this to be possible, but I've never tried it so I don't know for sure :)
You can definitely route the digital signals internally, and software can control the digital components; Thus, I would expect it to work like you asked.
I had some time to try what I wondered in my post and it did not seem to work. There may be another combination of settings that works. I think with the control registers being a limited resource, it would be nice to be able to do this, in particular when you only need one or two bits. If the fitter could split/share bits in a control register, it would be less of an issue, but that does not seem to be the case. For example, the ISP component in bidir mode appears to use a single bit control register. I was working on a design that added logic around it, and needed a couple more bits. It would have been nice to use a segment of the same register, but there does not seem to be possible. I am sure there must be some tricks somewhere in the huge pile of PSoC things I do not know...
Hmmm, that's a bummer :(
Well, something to find out in the future :)