cross-arch building picobsd/nanobsd images ?

Hi,
recently I have tried to build picobsd image for a different
architecture than the current one, with only partial success.

In particular, three weeks ago i committed some changes to the
picobsd script so now i can build working amd64 images on amd64.
However when i try a cross build (e.g. i386 image on an amd64 host)
the kernel stops right after trying to mount the root partition.
The error message is the following:

    ...
    Timecounter "TSC" frequency 1858691100 Hz quality 800
    Trying to mount root from ufs:/dev/md0 []...
    panic: mutex Giant owned at .../sys/kern/kern_exit.c:128
    cpuid = 0
    KDB: enter: panic
    [ thread pid 1 tid 100001 ]
    Stopped at      kdb_enter+0x3b: movl    $0,kdb_why
    db>

The backtrace indicates the following (i omit the numbers, as
i am manually copying the text)

    kdb_enter
    panic
    _mtx_assert
    exit1
    kern_execve
    sys_execve
    exec_shell_imgact
    fork_exit
    fork_trampoline
    --- trap 0, eip = 0, esp = 0xc3708d60, ebp = 0 ---

any idea on what could be going wrong ?

On a related topic, does anyone have experience on cross-building
nanobsd images ?

thanks
luigi