Maybe you could just do part of the island?
There was a project ages ago to make part of Plunder Island (pictured above) as a UT2004 map, where basically you were kept to a fairly linear area (eg: a single path through forest) but the illusion of being on the full island and being able to go all over it.
Alternatively, you could make it so you can see the more detailed parts of the island (eg: the town in the island pictured above) but not actually go into them, unless you really want to produce all that custom content.
It'd be an epic accomplishment, but it'd be damned fun. Monkey Island itself would probably be the easiest as it's mostly vegetation, beach, cliffs, etc.
It'd really get your name out there anyway, which is good for a budding pro.
I reckon you'd get some amazing publicity from it.