bernds: I got that working yesterday. The disk images are stored in files iibt1.bin etc., these are encrypted zip files. No luck cracking these yet either with a brute-force program or by typing in "Tarjan" (hey GOG, this is a form of DRM!) As mentioned previously however, they do get simply extracted while playing, so you just have to start each of the classic games once from the menu of the main application. On my machine they ended up in my user's "AppData/Local/inXile entertainment/The Bard's Tale/" directory.
To run them with kegs, you'll have to rename (for example) config.kegs.bt1 to config.kegs, and edit it to make sure the paths are correct - for me this involved removing the "%s/" from the strings. The necessary ROM file is in the main installation, "Res/Sys/bttricode".
Once you've got all that copied to a more appropriate location and modified as described, you can just run kegs (xkegs in the case of Linux, haven't tried on Windows). It helps to have run these once from the main game, since it includes a slightly modified version of kegs that displays hints on screen (such as where the character disk is located, s5d2 in the case of BT1/BT2).
How did you make kegs work on Linux? There are some vague instructions about making some ROM images (
/), which I didn't understand at all. Also, did you build it from source?