Why don't you simply allow negative factors?

Bear in mind that this stuff in config.ini was never expected to be user-visible. We were intending to put a nice, simple, sensible UI in emplode.

Sure, but wouldn't it be easier to write from emplode, too? I mean a

printf(",%s=%d",tag,value);

is easier than the way you would be writing it now, right?
Also, it would be easier to read for you, too, when debugging.

cu,
sven