Hi Jason,
Thanks for bringing this to our attention. We have made some changes to the API recently in order to improve performance. While I can't say that I was able to make changes to these table via the API prior to the update, I can confirm that I am not able to do so at this time.
I tested importing a new record into the InventoryAdjustment table via the Import Export tool and was successful. This leads me to believe that you should be able to make changes to these tables via the API as well.
I've created a ticket for our development team to investigate this further. I'll post back once I have some more information or once this has been resolved. Sorry for any inconvenience this has caused.
--Audisho