I have an expression rule that implements a common component that is going to be

What context are you using? If it's for a user then the loggedinuser() function could be included in the expression

The context is more about the functionality that is being implemented in each form.
The common component allows the user to select a valid employee (based on a web service call). One form uses it without any extra validation, whilst another form uses it with additional constraints (such as whether that employee was already applied to another task). The extra validation is based on local variable data inside the calling form.

You could pass in an integer or string that tells the common component which form you are calling it from. This way you can have all the custom validation in the common component, and you just conditionally check some of it depending on the form-type parameter that was passed to it.

You won't be able to access and use the rule's local variables from outside the rule, so you'd have to implement all the constraints within the rule. My solution allows you to choose which constraints you'll actually validate.

Thanks for your suggestions guys. It sounds like there isn't an easy way to keep the specific logic outside of the custom component. I was hoping for a nice way to implement a callback or something similar.