Cascading Dropdown Workarounds?

This is how we do - create separate client list Dropdown fields and use display Logic to show a specific client list when user chooses a combination of Region and Division. A note to consider here is the chart reporting you might want in future. 
Eg., "Americas Client Names", "ANZ Client names" etc. 

Cascading drop-downs can be accomplished with the External Lookup fields, though they are definitely fiddly to configure. For example, in my instance we have a "Product Family" field, and I needed a second drop-down where the user could select which Product Type the Project corresponded to but I wanted them to only see Product Types that fell within the Product Family to avoid reporting errors. It takes 3 fields in that scenario to accomplish. 

1. Single-select dropdown for Product Family

2. Multi-select dropdown for Product Type

3. External Lookup field for Product Type

Fields #1 and #2 go on a Task-level form. Fields #1 and #3 go on a Project-level form.

Then I create what I call a 'Control Panel' project and create a single task for every item in my Product Family dropdown. The multi-select dropdown then has every option corresponding to that Product Family selected. Essentially I built a database in my database. 🙂  That project and it's data get hidden away where users can't find them, they're only useful to reference later in the Project-level form. The magic comes in field #3. It becomes an internal API where Workfront talks to itself, looking for task information from your special Control Panel project filtering for where the Product Family selection matches.

I can see a couple ways to accomplish your variant, though they depend on things like whether your client list is really Company records in WF and whether or not the associated division/region information is already available on their records.

One way to go about it is creating separate client list Dropdown fields and use display Logic to show a specific client list when user selects a combination of Region and Division.
Note: On the form you use the same label for ex "Client Name" for all client fields but the name is different for ex "NA Client Names", "Asia Client names" etc. That is very important in UI .

Then adding a calculate field on the customform which looks at all the individual Client dropdown fields((that uses multiple If statement) and display the value from the one that has value.
By adding a calculate field that looked at all the different drop downs and displayed the name of the Client in a single field helps to use this single field in report grouping and report charts.

IF((REGION="North America" && Division="A"),NA Client Names,IF((REGION="XYZ" && Division="B"), Asia Client Names))