To build write. the bootloader, you first need to have a cross compiler (see: HowToCompile choose "e" for cpu target option). Then do a regular bootloader build which will produce a bootloader.bin file. (see: HowToCompile, sansa fuze+'s build is number 64). Then you need to get a Fuze+ firmware update from sandisk, decompress it and run the following command:

sbtoelf firmware.sb fuze+_key_file.txt OUT.

where the sbtoelf tool can be found in the trunk in utils/sbtools/ along with fuze+_key_file.txt; this should produce lots of output and several files:

OUT.____.0.elf, OUT.____.1.elf, OUT.____.2.elf, OUT.____.3.elf, OUT.____.4.elf, OUT.host.0.elf, OUT.play.0.elf and OUT.play.1.elf

Delete all files except OUT.____.0.elf, OUT.____.1.elf and OUT.____.2.elf and copy your bootloader.bin file in the same directory. Then create a file named bootloader.db with the following content

sources
{
    stage0 = "OUT.____.0.elf";
    stage1 = "OUT.____.1.elf";
    stage2 = "OUT.____.2.elf";
    bootloader_bin = "bootloader.bin";
}

section('init')
{
    load stage0;
    call stage0;
    load stage1;
    call stage1;
    load stage2;
    call stage2;
    load bootloader_bin > 0x40000000;
    jump 0x40000000;
}

Finally run the following command:

elftosb bootloader.db fuze+_key_file.txt bootloader.sb

It should produce a bootloader.sb file which you can send to the device using the imx_hid_recovery_tool which can be found in the trunk using the following command (device in recovery mode, see SansaFuzePlus):

imx_hid_recovery 1024 bootloader.sb

this way no modification will be made to the device and original firmware, but you will be able to test the current port's state