Looking at the generated code, this is probably not possible.
Each context variable becomes a standard property in an auto-generated context class. The "Default Value" becomes the "InitialExpression" attribute of the property. This means the default is assigned by the underlying object framework upon instantiation of the context object rather than by the BPL engine itself.
Class Test2.NewProcess1.Context Extends Ens.BP.Context [ ClassType = persistent, CompileAfter = Test2.NewProcess1, GeneratedBy = Ens.BPL.Compiler.CLS, ProcedureBlock ]
Property testvar1 As %String(MAXLEN = 50) [ InitialExpression = "blah" ];