SQLAnywhere: How to configure length of message output to console/client when using isql
I found a solution at least for my basic issue, which is almost too simple: 

    -- ... some code
DECLARE @generatedProcedureCall LONG VARCHAR
SET @generatedProcedureCall = ... contents
SELECT @generatedProcedureCall


This is allowing me to print out >4k characters for e.g. debugging. 

When writing to file e.g. 

SELECT @generatedProcedureCall INTO #logfoo
UNLOAD #logfoo to 'some/file'


could do the trick. 

Maybe not the cleanest one, but as mentioned it solved at least my issue. 

The reasons seems to be, that print as well as message cannot handle string >4k of text. print was just failing with an error whereas message catted my string to 255 characters.