[syslinux] Bootable ISO with extra data outside the ISO?

[Bret Schuhmacher]

Sorry if this is a total noob question... I've searched the archives and
haven't turned up much.  Not turning up much on the web in general, either.

I'm working on a project and I'd like to create a Linux bootable ISO disc
with an extra chunk of data **outside** the ISO space.  This chunk of data
might be a 10MB tar/zip.  The goal would be to boot into Linux in RAM off
the disc and mount the extra chunk of data to access it.  Is this a hybrid
ISO?  I'm not asking for the command line recipe, but maybe a "yes, this is
simple, use xorriso and check out feature XXXX" would be helpful.

I need this because I want the Linux ISO to be constant and verifiable with
an SHA hash... the extra stuff will vary from customer to customer.

Thanks!

Bret