Well, I'm incurable.
FreeBSD10 has been "out" for 1 day and I just have to install it.

I've done this before on my current slackware14 install, but I failed to write down the steps.

I fired up gparted and squeezed out a ~100 G partition for the FreeBSD install (unformatted and labeled) but when I went to use "Manual Partitioning" I kind of got scared as I wasn't certain it was installing to ada0s4, the partition I want to dedicate to this task.

I saw nothing like this, it shows ada0s[1-4] where I took ada0s[1-3] to be my current slackware install.

My current layout says fdisk -l is:
on my 1 Tb SATA disk.

The Manual says
"A standard FreeBSD GPT installation uses at least three partitions:
Standard FreeBSD GPT Partitions
freebsd-boot - FreeBSD boot code.
freebsd-ufs - A FreeBSD UFS filesystem.
freebsd-swap - FreeBSD swap space.

but a df -hT in my FreeBSD VM shows:
Do I really next an Extended Volume with 3 partitions for this task?
I know I don't want to install anything to the MBR, as I have used and plan to use my existing lilo (LILO version 23.2) from the slackware install.
How should I proceed?

Thanks!

You need a separate disk with a GUID parition table (GPT) on it. FreeBSD then puts the first boot loader stage into the PMBR. You need the boot partition for the second stage. This has nothing to do with extended volumes. In a future version FreeBSD most likely will only support EFI booting and drop 32 bit (IA32) support completely, just as RHEL 7 did.

The A5 partition on a MBR disk is supposed to contain a legacy BSD disklabel. I don't know, if the new FreeBSD 10 installer still supports that, but I'm sure bsdlabel support in FreeBSD itself will go away sooner or later, so I would not recommend using it.

TL;DR: Use a separate disk for installing and booting FreeBSD.

So the successful install of a dual boot on FreeBSD 9.2 and slackware on the same disk cannot be duplicated with FreeBSD10 and slackware14?

If you manually assign the partitions rather than have the automatic tool do it for you, you can use the MBR partition and rerun your a boot loader like GRUB to detect and assign a bootmenu entry for FreeBSD, or you can use the automatic tool which will replace the boot loader with boot0 and you can use boot0 to boot Slackware and FreeBSD alongside each other.

Nothing has been removed from FreeBSD 10's installer. Check the FreeBSD handbook for more information.

I suppose here too, that if you change to boot Slackware from EFI and GPT in the future, next release perhaps, it will make this sort of uncontrollable urge easier to satisfy. I've been thinking of doing the same thing, but will probably try a VM first.

Thanks.
I just ordered another Drive.

"640 k ought to be enough for anybody"?

My cheat sheet for installing FreeBSD 10 on Virtualbox

HA!

Did it. The short and sweet of it goes like this:
Boot the FreeBSD 10.0-RELEASE and select Multi-User
Install
set the hostname
Distribution Select -- select src!
Manual Partitioning
...Partition Editor
......I removed ada0s4a and ada0s4b (Created yesterday with PC-BSD installation)
...Create on ada0s4.... set mount point as /
Install.... set root password etc...
reboot when done.

No mention of MBR or anything, so I modified Slackware's /etc/lilo.conf and added
Reboot into FreeBSD10 and
Enjoy the Goodness.

I suppose having another 1 Tb drive wouldn't hurt to have around.