Convert GPT disk to MBR disk in-place?

Is there any way to reliably convert a disk from GPT to MBR in-place?
(i.e. without having to backup/restore any partitions.)

This conversion can be done with my gdisk program, The actual GPT-to-MBR conversion isn't all that complex (to the end user), although there are some significant caveats, such as:

• If there are more than four partitions, some of them will have to be logical partitions under MBR, and those have spacing requirements that may not be met in the original. Thus, it may not be possible to convert all the partitions without resizing some of them.
• Some GPT data, such as partition names, will be lost.
• If the disk is over 2TiB (assuming 512-byte sectors), partitions that extend beyond the 2TiB mark won't be converted. MBR has a 2TiB limit on partition sizes and start points, which is one of the main reason it's being put out to pasture; it's just not useful on larger disks today.
• If the disk is a boot disk, you may need to re-install your boot loader. This will be a real pain with Windows, but is likely to be easier with Linux.

The gdisk documentation covers the process in detail. The bigger question, though, is: why? There are legitimate reasons to want to do this conversion, but there are also a lot of cases when such a conversion would be ill-conceived. Knowing why you want to do the conversion would enable me to better advice you about whether converting to MBR makes sense or if there's some better way to achieve your ultimate goal.

MiniTool Partition Wizard can do this.
I just tested in both directions (MBR <-> GPT) and it did the job in ~10 seconds.