I toyed with the idea of allowing this, in bitmaps, it's pretty easy and efficient to find contiguous areas with bit twiddling hacks, for example

//assume free map is the bitmap where 1 means free

uint32_t free_map;

uint32_t free_map_2 = (free_map & (free_map >> 1)); // so on and so forth

I haven't really done anything like this yet, it has certain disadvantages, but you can pack multiple size classes into the same bitmap, you do a bit more work during alloc and resolving interior pointers is a bit more costly (if you have those), in exchange for having less size classes.

Sure, to find contiguous chunks of 6 slots within a single word, you can do

    t &= t << 1;
    t &= t << 2;
    t &= t << 2;

Should I assume from your use of uint32_t that you're targeting embedded ARM microcontrollers?