You can use Field read only user property. If u choose 
priortiy filed as open, every fields should become read 
only except priority field. I achieved this concept through 
Field Read only property......

Create a new Calculated field with the BC like:

Test Field = 
IIf([priority] = LookupValue("PRIORITY","Closed")), "Y","N")

Then with the BC User Property write a Property:

          Name           Value
BC Read Only Field      Test Field   

So once the value of the priority is set to closed, the 
whole of the record becomes read-only now.

This can be done through "bc read only field" user property.
Crate a calculated field and in calculated value, just give
the logial expression accordint to ur recuriment. This c.f
should be assigned to bc read only field user property...