I've looked for the actual images before and never was able to find them anywhere. In the Danish family that I tracked, the ID Codes were in numerical order starting with the father. Since you've made a list of everyone on the ship, you might want to look for people with adjacent IDkode numbers to William.
Edit: Looking at the IDkode a little more, if he was traveling with someone not named Meilstrup, the codes might not be exactly sequential as the M in D8991M1603 is for the surname. However, the last four digits stand for page 16, line 3, so I'd think the person/persons he was traveling with would be on the same page.
Edit: Nope. The pages are by the first letter of the surname, so an IDkode with M will only have people whose surname begins with M.
Edit: After further review, families with different surnames may be kept together on the same page. For example, Lars Rasmussen's adult children all have the last name Larsen, but they are listed on the same page with him and indexed under IDkode "R": https://www.familysearch.org/ark:/61903/3:1:3Q9M-C332-87DM-G