Marc is actually on vacation in Turkey. I've been attempting to pseudo-cover for him (though I don't think anyone can live up to Marc's level). I saw this post this morning but didn't reply as it looked like you've had a lot of dialog with Marc in the
past, and he sometimes logs on to answer some of the posts (despite the fact that he's supposed to separate himself from the phone and laptop on a vacation!!).
I'm trying to more fully understand what you're doing so I can try and point you in the right direction. You're never going to replace the SharePoint ID field. You can certainly create a new column for IDs, but SharePoint is never going to use that column
as the primary key; lookups and such are always going to resolve back to the SharePoint ID column.
Are we talking form/data entry type uses for your wanting to use the other ID, or more the displaying of the data?
The reason I ask is that I would think you could accomplish the form stuff without the use of SPDisplayRelatedInfo (unless I'm misunderstanding your goals). Simply add a new lookup column back to your real ID list, and use some simple jQuery to hide the
old SharePoint fields (as applicable). If you're looking for more the displaying of data, with a large result set, repeated web service calls probably aren't very efficient. You'd probably be better off linking the data sources and using a DVWP to display
If you'd like to post some more information I'll reply with some updated options for you.