What I want is more or less the exact behavior of xdg-open, but I want
to be able to check the actual command (e.g. "evince", not just
"xdg-open") before pressing RET.
I don't understand why this is useful. If you want a particular command for every file of a type, configure it; if you want a particular command this time, specify it.
I also want to be able to override xdg-open with customizations inside
Emacs.
This just adds complexity to the system.
Please bare with me as I'm trying to understand.
Is your preference still to just run "xdg-open", as you explained in
your original bug report?
Do you agree that if we could get my idea to work, you could have your
cake and eat it too?
No, because I was hoping we could make the cake smaller.
IOW, do you disagree with the approach because you
think it is infeasible to make it work well, or because you don't think
that the feature sounds useful? Or do you merely disagree with using
mailcap to implement it?
I agree with Eli that mailcap is not a good match; also it's widely considered deprecated (for example, GNOME's mechanism doesn't use it), so its configuration is likely to be out of date.
Mostly, I am afraid that a suggestion which satisfies everyone in terms of functionality will simply pile on more features to Emacs, in an area which is already too complicated. I don't see why launching external applications can't be left to the operating environment, as all environments now have mature systems for doing this. If someone such as you wants more complex functionality, that can be served by a third-party package; it doesn't need to be built in to Emacs.