Big Corn Island has a local port office where foreign yachts must contact in person, then visit the Corn Islands. Just let them know when you're leaving again. That sounds like what you want. But international clearance into all of Nicaragua
cannot be done at the Corn Islands; visitors must go to the mainland for that. Warning, from Roatan southbound, if you do aim for the Corn Islands, stay well east of Miskito cays and channel due to local bandito trouble in the Moskitos.
: the Central American Route" contains a chapter on southwest Caribbean
that covers both Corn Islands plus San Andres, Providencia, the Moskito Channel (avoid that), the Nicaragua
Banks, Cayo Vivorillo, etc. It's my book, we're constantly updating it, and it's sold on Amazon. Just sayin, Corns are fine, Moskitos are not.