currently, ReactOS mount manager acts like the Windows one and so does not seem to support partitionless (aka the oldie "superfloppy" specification of creating the file system from the sector 0) non-fat16 disks. That restricts the ability to access disk with existing data formatted in other O/S.
With the help of hbelusca, patching disksup.c of ntoskrnl in order to relax the superfloppy algorithm enabled me to use these disks without having to re-layout them. In order to not break compatibility, a boot flag (e.g. tried with /SUPERFLOPPYRELAXED for a personal build) or a registry key could be considered.
Partitionless disk are present on NASes, on filesystem with internal volume management (e.g. btrfs) and general fixed disk too so it could be an addition to be considered and are supported on GNU/Linux systems and mobile oses (e.g. iOS and Android).