Breakpoint files are not in any way functionally or semantically connected to a soundfile (it would be very common for the same file to be used with many different soundfiles). If the soundfile happens to be longer than the final duration of a breakpoint file, the final value is sustained. All CDP programs parse the breakpoint file to ensure time values are always increasing. I suppose, given a basic utility to report a duration, a shell script, perl program etc could be used to detect a placeholder string such as "VAL" and replace it with a value extracted from the console output of "sfprops" or "sndinfo len", or from whatever new utility was written to do the job.