Well, this was an eye-opening exercise. It appears that Digia has been trying hard to make their Qt 5.x series backwards-compatible, but not nearly hard enough.

1) I tried downloading the 5.x SDK, and that works fine, so far as it goes; unfortunately, the maintenance tool is broken (and has apparently been broken for months), so there is no easy way to download older libraries or other toolchains. There are lots of complaints about it, but nobody seems to have an answer yet.

2) Digia provides the 4.x libraries, but leaves it up to you to figure out how to integrate them into a 5.x installation. Without the maintenance tool's package manager, I would imagine that this will require a lot of head-scratching... (Also, I haven't yet been able to find any archived toolchains.)

3) Digia does _not_ provide any of the older SDKs!!! I searched and searched, and all I could find were links to Nokia's websites. Unfortunately, it appears that Nokia has decided to terminate with extreme prejudice any Qt or Maemo or Meego related software on their site.

So, things look a lot worse in the Qt world than I had expected. However, folks on this website have already bumped into this problem. There, they've found someone still supplying the last Nokia Qt SDK:

http://jenkins.taulabs.org/distfiles/

However, that only has the Harmattan toolchain. Still, get it while it's hot...

I now have absolutely no clue where to get a copy of the Qt Maemo toolchain. (I'm going to have to start making backups of mine! Maybe I can find my original SDK installer as well...)