Vertrees 2001 (where it is listed without description) uses the spelling 'Rokugatsuen nishiki'. When I Google Rokugatsuen I get references (on the first page, anyway) only to this maple, and am asked if I want Rokugatsu instead. When I look at the first page of hits for that word it includes, in addition to mentions of movie titles and people with that name, comments about it standing for the month of June. So, I'm thinking maybe the name of the maple equates to something like "June Brocade". If not that, maybe it commemorates someone, as in "Rokugatsu('s) Brocade". If en works like a suffix in this instance (and is not a mis-spelling) that would certainly pertain.