RB: Thanks for the tip! :-)

For the interested reader I can point out that the layout_filename really IS the unquoted symbolname created by concatenating "layout_" and the prefix of the layout filename, in my case layout_mp68a and layout_md6802. The symbol is created by the make system by parsing the layout file into the .lh file. There is no compile error trying a string or some other ignorance. The next thing to lookout for is the -debug switch that seems to block the late binding of the 7seg IF more than one machine driver is provided in the build and the selection menu appears. Then you need to explicit say which driver to debug on the command line, for example:

./didact64d mp68a -window -debug