After creating a somewhat dubious MBR in disk images formatted by rexxfat I've now also added a dummy VBR doing more than only
INT 18h (hex. CD 18). For an ordinary FAT12, FAT16, or FAT32 volume this VBR displays the OEM Id. (8 characters at offset 3 of the VBR), the file system type string (8 characters at offset -8 from the begin of the VBR code), the volume label (11 char.s, offset -19, default NO␠NAME␠␠␠␠), and the wannabe-unique volume Id. (4 bytes shown as 8 little endian hex. digits, offset -23).
Wikipedia claims that the file system type string and the volume label only exist for an extended boot signature introduced by 29h at offset -24 from the begin of the code. In other words, for 28h only the volume id. can be expected to exist, and for anything else these 1+4+11+8 bytes won't exist at all, notably on media formatted with PC DOS 3.x or earlier. The rexxfat dummy VBR code is designed to work for any 29h FAT layout with sector size 256, 512, or more. Check out the script for the OpenWatcom WASM assembler MBR and VBR sources in two REXX comments.
Apparently Darwin VBRs expect that SI points to their partition table entry in the MBR, and just in case rexxfat now guarantees that when it boots an active primary partition. Looking at a Darwin MBR source I think this is only used in its magic to boot a logical disk in an extended partition, i.e., the relative number of hidden sectors from the begin of the EBR to the logical disk VBR is updated on the fly to yield the absolute number of hidden sectors from MBR to VBR, and that's of course the LBA. This won't help normal FAT VBR code expecting the number of hidden sectors in its own BPB, and the rexxfat MBR limits its efforts to initialize SI as expected for primary partitions. Patching loaded FAT VBRs in an extended partition is left as an exercise for the EBR code — and so far rexxfat only formats one primary partition with a FAT. More convoluted tasks are a job for GRUB, BCDEDIT, diskpart, parted, or what you have.