Well just because the HFP needs the railing doesn't mean Mugen needs one too. I'm guessing it doesn't need it, but you can ask ppl who put on the JDM Mugen Civic skirts, which should be the same as the Mugen Civic skirts.
Edit: I was digging around for this and found it. I'm pretty certain it won't need it. Here's the Honda parts listing for the skirt, and the hardware mounting kit (3) shows no railing.
