I also just found out that at adapter version 2.3 (LX version 8.3.5) there's no access to the User Fields on ACP400D4-01, (that's only available in version 3.0/LX 8.4.x) so that pretty much means it's not going to work for us at any rate.
It appears that the only use case is for items on an LX purchase order, and even then I'm not sure how you go about expensing those correctly given that you can't specify a GL account/alias or override the tax rate.