Is it possible with some rather small changes to redisplay make it per newline property, so you can specify which line to truncate and to which apply buffer setting?
Here is the example:
(setq truncate-lines nil)
(insert "<very long line>"
(propertize "\n" 'truncate-lines t)) ; this line will be truncated
(insert "<another very long line>" "\n") ; this line will be wrapped
What do you think about this feature?