Region Free with circumvention utilities
The Mega-CD has region coding by default on every disc, but fortunately this is fairly easy to circumvent. Cartridges were released at the time the system was active that let you select the region, letting most discs boot out of area, and there’s even one available commercially today, from http://products.genny4ever.net/megacart_v1x.html – the Megacart V1.x, also a flashcart for Mega Drive ROMs.
A note, however – the Mega CD is a lot more sensitive to PAL/NTSC Hz timing issues than the Mega Drive is, and some more problematic titles may not boot at all. With time and results, a list may be assembled here.