Because I'd have to re-install it after each change to the code.

Actually, you don't. You need data files to be installed, yes, but you don't need the actual executable to be. To run the in-build-directory binary, use libtool (as most of the code is now in libgeany): libtool --mode=execute src/geany.

Could the data folder be treated like the config folder, a default value you can override on the command line?

That wouldn't make much sense, the location of Geany's data files are not something a user is supposed to want to change depending on the runs. It's mostly part of Geany itself, not a runtime choice. Basically, if you want to do that you're doing something wrong.

With that there should be only dealing with libgeany.so.0 left to make a portable version (launched through a bash script setting relative path).

Ah, relocatable executable on Linux. Well, fortunately for you, we do already have support for this, you just have to enable it with --enable-binreloc option on ./configure call. You still have to install the resulting build, but you'll be able to move the tree around (and can very well install wherever you want, either with --prefix or DESTDIR).
Binary relocation isn't very well tested though, so feel free to report issues with it.

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