Maybe there could be a standard log file path for each platform which could be
the default for logfile=''. Logging to stderr could be selected with
logfile=stderr.
The way you suggest, there would still be the possibility of specifying different log locations for every configuration, but now there is also the added confusion of a "stderr" switch. I think it would be less confusing to implement the option as you first suggested, where std output is the default and the logfile= option is used only to define a log path to override stderr. Maybe a default log file location would be useful if the option is found in a configuration but the path is blank e.g. "logfile=".
1: before default config file is loaded 2: after default.uae is loaded 3: after user-selected configuration is selected 4+: if user selects another configuration on the fly.