WP has definitively retained the MT post IDs and spot checking indicates that they are addressable via URL in the form of <domain>/?p=<1 to 4-digit post ID>.

So I just need to set up a rewrite rule in .htaccess – there are some varying approaches to managing the unpadded ID form – MT output six-character ID strings so post ID 39 was incorporated into the URL as 000039 and 5769 was 005769.