Are you asking about calling SERVERCLASS_SEND_ from COBOL code to send a message larger than 32KB? Or are you asking whether it is possible for a COBOL server to read a request message larger than 32KB from its $RECEIVE?
While I don't have any definite knowledge about it, I see no reason to doubt that the former would work, unless COBOL doesn't even allow you to declare a Working-Storage record larger than 32KB. The Limits section of the manual claims there is no limit on the size of a record in Extended-Storage or Working-Storage, but I wonder how large of a record has been tested. It probably is possible to declare a record larger than 32KB, but one as large as 2MB might not work, though the documentation says it will.
The latter might be a problem. The most recent version of the ECOBOL manual still says that the maximum record size for a file assigned to $RECEIVE is 32KB. It might be that the manual just hasn't yet been updated. I do not have access to the softdocs (at least the last time I tried to look for a softdoc, they were protected by a login I don't have). If you can get the most recent ECOBOL softdocs, that would be a place to look. Otherwise, if nobody else here knows the answer, I suggest you call the GCSC, or whatever it is called today, to ask the question.
If you were willing to make the server open $RECEIVE and read and reply to its $RECEIVE messages by using ENTER to call the Guardian FILE_OPEN_, READX, and REPLYX procs, rather than using an FD assigned to $RECEIVE, then that ought to work for the large message sizes without a problem. But I assume that isn't what you were asking about.