System fails to boot with root on ZFS

I’d installed FreeBSD on my ThinkPad X61s when the head branch of the source tree was at 9-CURRENT, multi-booting it with Windows 7 & OpenBSD.
At the time I was not aware that it was possible to boot FreeBSD from a root file system on a ZFS volume from a disk partitioned using a MBR scheme. Instead, I opted to store /boot on a UFS filesystem.
This install existed for a couple of years, the ThinkPad got a roasting every once in a while to build a new release to install for updates. At some point support for 4K sectors in ZFS was improved, zpool status began to report degraded performance as the disk had been using 512byte sectors where in fact it could support 4K sized sectors.

Eventually, I deleted the existing slices in the FreeBSD partition & attempted to reinstall but found this time the system would not boot.
Booting from the install CD & issuing zpool import reported the new pool & old pool from the previous install.
Destroying a pool before deleting slices stopped this problem from re-occurring but the system still wouldn’t boot from a ZFS volume on a MBR partition.
The next step was to see if things would work if the whole disk was dedicated to FreeBSD, with a GPT partition scheme, things worked but switching to MBR, again, it failed to boot, hanging at a flashing cursor.
Over the next four months, many installs were attempted. On a MBR partitioned disk
FreeBSD failed to boot but PCBSD could by using GRUB.

I stopped trying any further at this point & took a break from it, one thing that had been raised at BSDCan was the possibility it could be lingering metadata, I’d thought as zpool(8) wasn’t reporting any existing pools when asked to import that this wasn’t the case. To give the benefit of a doubt, I ran dd on the disk with no difference in result.
This approach to clearing old pools seemed a little rough so over the weekend I looked into what other options are available.

The zpool(8) man page documents the labelclear option as

zpool labelclear [-f] device

Removes ZFS label information from the specified device. The device
must not be part of an active pool configuration.

-v Treat exported or foreign devices as inactive.

I still had the FreeBSD snapshot from the last attempt which I booted the X61s with, headed to the shell, deleted the existing partitions & issued
zpool labelclear -f /dev/ada0

Everything worked as intended after that.

Thanks to Allan Jude & everyone who chipped in at BSDCan.
Through the trial of getting this working Allan added the option to use a BSD partition scheme to the FreeBSD installer as well as MBR & GPT, which was previously unavailable.