... Well. It would certainly be POSSIBLE, but IS IT A GOOD IDEA?

Most "Harem-animes" so to say focus on the time between the male character and each female one at a time. That, of course, will get trickier in a roleplay with all the other characters who have to be doing something else at the same time. They need something to do, too, after all. As ELGainsborough says, it'd help to have a story that focuses on something else than the romance to give them something to do. If you somehow want to focus on the Romance...

... Hah. Suddenly I got the image in my head of a roleplay where all players play girls who are in love with a single NPC boy, and they all need to do their best to snare his heart, competing with one another while the NPC is a total buffoon when it comes to awareness, anime style. Not sure how well it'd work, but in my head it's a fun idea. Haha. In any case. That was my immediate thought.