Hi,
In order to be able to insert new creatures inside the game, I've heavily modified the Nif Plugin to use Havok entities instead of the bhkRigidBody modifier used for normal collisions. It also support havok constraint, both import and export.
source available here: https://github.com/aerisarn/nif-havok-max2010-plugin
COMPATIBLE ONLY WITH MAX2010, HCT 2010.2.0 MUST BE INSTALLED (not included)
Current shortcomings:
- doesn't support Phantoms, will be added
- Unknown data is zeroed except for unknown int 2 which is 1 by default
- May break some other things related to non-creature exporting
Use the following settings to import/export skeletons:

(ignore paths of course)

| Attachment | Size |
|---|---|
| 1.13 MB |

HCT:
http://www.sonicgenerationsmoddingtools.com/havok-content-to...
Just wanted to post a quick update on my attempt. Looks pretty great overall, aside from a few unknown nodes. It's hella fast, and the export/import works flawlessly:
@Mace, supremely tempted to post your second screenshot to the Skywind subreddit as "New cliffracer model finalized"...