Posted at CrossFitCS.com but copied here for anyone that might benefit.
Aaron, here are a couple of thoughts on what you could do to work up to HSPU's as well as sub's for the WOD.
Firstly, can you kick up into a hand stand against a wall? If not post back and I will fill you in when I have some other stuff to hand.
If so, your sub for HSPU's could be kicking up into a handstand and holding it tight for two or three seconds, come down then repeat for reps. Sounds easy but can be a real cardio smoker!
Next, to work the movement itself. Set up a stack of thin books (about half an inch each) in between your hands below where your head will be. To start with allow about an inch free space below the level of your head. Now kick up into a handstand and ever so slightly bend the elbows and straighten them again. DON'T even go for the stack - just get the confidence to bend and straighten your arms.
IF you are comfortable with that go to the stack (gently!!) then push up. Once you are comfortable with sets of 5 to that depth, remove one book and start again.
IF YOU CAN'T control the movement even a little bit then find something like a table top that you can rest your feet on. Get into an inclined press-up position with your feet on the table. From there walk your hands towards the table until you are bent in an L shape with your body and arms near to vertical. Get comfy then try the elbow bend and straighten process again.. If you are good to go then use the book idea mentioned above.
If not then work on incline press-ups and bench/ bar/ ring dips to further strengthen your arms and shoulders.
Regardless of what level you are at you should also be spending at least a few minutes a day inverted even if it is just static holds or even kick-up practice. Not only will you become for comfortable with this down-under perspective but your shoulders will gain strength and flexibility.
Hope this helps.