Why would your zpool be in any danger from the upgrade? If you upgrade via freebsd-update and have renamed your custom kernel (and not named it GENERIC), then

Why would your zpool be in any danger from the upgrade? If you upgrade via freebsd-update and have renamed your custom kernel (and not named it GENERIC), then freebsd-update will tell you when to build and install a new version of your kernel. If you kept GENERIC as the name of your custom kernel, which is a really really bad idea, then freebsd-update will probably still replace it with a vanilla kernel, haven't checked that in a while. In that case either rebuild your 10.3 kernel again with a fixed name, or upgrade to 11 from source.