After poking around in the code, and finding what to look for in sendmessage.php, I found that there is indeed code in messhandler.php to check if a user has their PM acceptance disabled.
The /languages/english/messhandler.php file is however missing the language text for the "NO_PRIVATE" result. This causes an annoyingly uninformative "We Cannot Proceed" page with no hint as to why the software can't proceed.
Adding
$ubbt_lang['NO_PRIVATE'] = "This user is not accepting private messages.";
back into /languages/english/messhandler.php fixes this though, and will then display a message that the user isn't accepting PMs in the case that a user tries to respond to a PM from someone who isn't accepting them.
Just to verify that I wasn't going nuts after checking my fresh copy, I unzipped the 6.5.1 distribution file I downloaded Wednesday morning to check, and that line is missing from the messhandler.php language string file found in it. Looks like this was missed and is being distributed like that.
