Hi Paul
The function behind that WS function is the function get_courses() in server/course/externallib.php. You'll see that the function returns a specific set of fields (which I believe are the core fields). Custom fields are not included.
What you are after seems like it should be expected behaviour, so you might consider submitting a feature request.
In case someone from Totara is reading this, there is a bug the file I referred to above (in TXP15, I haven't checked other versions).
The file is littered with SQL statements like 'SELECT <blah> FROM "ttr_course" ...'
Hard-coding the table prefix in SQL queries will make affected functions have unexpected results on sites that don't use 'ttr_' as the prefix.