If you have absolutely no option other then using 2011.x then you should install 2010.2 and copy/paste its Havok folder somewhere it will not get overridden then install 2011.x and when you export, export as Packfile - XML then see if you can get the 2010.2 standalonefilemanager or assetcc2 to convert the file to the 2010.2 format so it will actually be compatible with Skyrim.
That's not a bad idea, thanks.
Right now here's where my workaround stands:
Step 1: Export from Max as a KF via NIFTools exporter
Step 2: Convert the KT into an XML HKX via hkxcmd
Step 3: Export from Max as an XML HKX via HCT 2011.3. I had to delete the two camera objects from the scene before doing this: they weren't showing up in the KT version. I hope this won't cause any problems.
Step 4: Copy the "FloatBlockOffsets" and "Data" Arrays from the HKX exported from max via text editor and paste them over those arrays in the HKX converted from the KF. "Data" is the one with the HUGE wall of numbers, and FloatBlockOffset comes right before it. Make sure you grab the "size" field from before the data array as well.
This leaves me with an XML HKX that the Havok Preview Tool, at least, thinks is just fine. The animation is entirely intact (the KF -> HKX animation loses too many bones and keyframes for some reason, and can't figure out IK animation to save its life) In theory, so should the CK and Skyrim, but since I have no way of repacking the file without changing it to the 2011.3 version, there's no way for me to get it there.
The 2011.3 and 2010.2 HKX formats are very similar. Virtually the only change is as I mentioned above, the fact that in the 2010 version the "AnnontationTracks" array is full of [ref]s that point to each track as an object, which is defined later in the document. In the 2011 version, the tracks are defined right there in the array with [struc]s. Since the actual animation is all stored in the "data" array as far as I can tell, a program which converts one to the other shouldn't be hard. (but sadly, I'm not a programmer.)