Way to implement a compound type? (enum with fallback u32)

[RocketMaDev]

Sometimes a int field have some sorts of special constants, and I would like to bind them to enums, while the rest numbers keep the normal form. For example, type SECT takes a u32 value, and a special value 0xffffffff means "END", how to control its display, so that when a field is not (u32)-1, display 0xffffffff, and when it's (u32)-1, display as END?

[RocketMaDev]

I try to write like:

enum SpecialSECT : u32 {
    MAXREGSECT = 0xfffffffa, // Maximum regular sector number
    RESERVED   = 0xfffffffb,
    DIFSECT    = 0xfffffffc, // Specifies a DIFAT
    FATSECT    = 0xfffffffd, // Specifies a FAT
    ENDOFCHAIN = 0xfffffffe, // End of a linked chain of sectors
    FREESECT   = 0xffffffff, // Not allocated sector
};
fn SECTconsts(u32 v) {
    if (v > 0xfffffff9)
        return SpecialSECT(v);
    return u32(v);
};
using SECT = u32 [[format("SECTconsts")]];

But SpecialSECT is treated as a function, rather than a type, so I can not cast.