So my "port" to VC++ 7.1 was nearly done, only in two instances I could not work around some constructions in the sources that my old compiler just would not understand.
After pestering the KeePass forum with my problems I immediately got responses from Bill Rubin and Dominik Reichl - himself; accepting my proposed changes into the KeePass code. - That's the developer support I had hoped for! :-)
Bill then friendly but repeatedly mocked my "antique" development tools, and after some time I decided to try out one of the "Express" editions of Visual C++ that are available for free - something I once thought about already some time ago.
My first attempt was to use C++ 2010 Express: The IDE is new and sleek (although I like the more toned-down older ones better), but there was a slight problem: The Express editions don't include MFC, and the new compiler would react "violently" to the MFC 7.1 header files of my installed Visual Studio .NET 2003.
I then hoped that the 2008 version would (still) be able to compile them, and also got a C++ 2008 Express. With the same results.
But that isn't too much of a problem, as the plugin does not use MFC (and probably will never do), and I need a debuggable KeePass only for, well, debugging - and can build this with the Visual Studio 2008 I use at work.
So I - for now - abandon the "backporting" effort and will use Visual C++ 2008 for the project - after all, this is the "official" compiler for the KeePass codebase.
There are only a handful "bounds-checking interfaces" missing for the Visual C++ 7.1 build however, so I might add these functions some day anyway - just for fun.