Best thought I have for this condition is to create a system module that uses the "user" as a dimension and contains all of your properties, including a line item that is list formatted to the list that you'll need to use for the valid users in your dependent drop down.
The system module can be converted to a calculation module if you want, so you can effectively map the user list to your user-defined list.
Let me know if you'd like an example - happy to try to build that - but one way or the other, it seems to me you're going to need a 1:1 match between the current user and the user-defined list you're working with (by department).
In this example I want the USER to be able to view multiple items on the Department list so not sure the USER list with Department List as a line item will achieve this, as this would only allow for 1 item to be mapped through
Effectively I'm trying to recreate Selective Access with out activating Selective Access but it may just be a lot simpler to use it
Yes that does work as I initially thought it might, I have no idea what I was doing the other day to result in a fail test but it's working now as expected (you do not require the USER dimension in the current module to filter it by USER)