Just did a little testing. (progression.xml)
You can add a Level 0 bonus:
<passive_effect name="CarryCapacity" operation="base_add" value="9" level="0"/>
To the perkPackMule and adjust your values from there for the rest of the levels.
In my case, I added a baseline of 9 more slots, then adjust the rest so they open up as I level the skill more.
I imagine if you put in the right values (would think 200-18 = 182 should give you a 'full' from the start as long as you add those values to all the levels (or never unlock one)
Could you hook me up with that 200 inv setup?
(This is NOT 100% tested... Just know it removed the encumbrance icons form the slots)