Firstly, apologies for the necro.
I was looking for a way to get a complete Attract Mode Romlist while having merged ROM sets for MAME and came across this post.
I'm not quite sure if the command line that raygun provided actually worked in a previous version, however I can confirm that it does not work as you would expect with the current 2.0 release candidate.
The following command line generates a list of 10954 entries (current 0.171 build of MAME).
attract --build-romlist mame --full
This only includes parent ROMs, not the clones inside the merged ROM files.
However, if you manually create a MAME listxml file first using the following command line:
mame -listxml > mamelist.xml
then use the import-romlist option instead of build-romlist, it will generate a full list of 32965 entries.
attract --import-romlist mamelist.xml mame --full
Not quite sure why manually creating a listxml file and then importing is treated differently than letting attractmode run the mame executable to generate the listxml automatically, it seems like it's applying a hardcoded filter to discard clones (or it's ignoring the --full argument).
Anyway, I hope this helps any other folks that maintain merged sets!