Your existing rule engine tool supports your web application, which means it meets your needs already. You can use other "Rule Engine" like MS work flow, but IMO it can also end with a hard to maitain situation.
Let's say there is registration portal. It collects general user infomation and save them into database. Simple. we build one protal for one client with several ASCXs and Rules.Then for another client,we add more rules and more controls to these ASCXs. Working in this way, sooner or later we will reach the final straw client. At that time the code base is hard to maitain and devs lost themselves in lots of rules. It is what happened to me.
So to me, it is not about which Rule engine to use.
Then How?
I have raised a question, and one of the answer makes sense to me( thought not a picked answer). In this answer, the guy mentioned what kind of company you are. In your question it is more like which department you are or do you want to seperate your dev teams.
If you are in a archetect teams, build a framework with a rule engine. Create a basic registraion portal as a sample portal.Make DAO,BO decoupled with UI (Seperate layers).
If you are in a customise teams, create customised user control (dont reuse these user control in basic version). What you will recreate is just UI, you can still use DAO,BO as they are not defined in user control, they are at other layers. In this way you get the freedom to define your client specified rules without worring about contaminating other clients rules or introducing new bugs to other client's registrations.
Just realise it is not like an answer to your question. Anyway it is my thoughts after limited xp of working on a engine rule based ,multi-clients web application.