Thanks to all for putting together such a handy set of tools.
I noticed that the memtest86p 'kernel' doesn't work (can't be found by the isolinux/syslinux loader). This is due to a naming problem: isolinux can only see the ISO9660 name, not the Joliet/RockRidge long name. (Much the same holds true for syslinux, substitute 8.3 MSDOS and VFAT for ISO9660 and Joliet/RR). See syslinux.zytor.com/errors.php#naming for more info.
i.e. the VFAT filename 'memtest86p' is too long as an ISO9660 and DOS 8.3 name. Renaming it to memtest (and updating syslinux.conf to use the shorter name) works a charm for both.