It is always a good idea to hide your membership-related activity behind a service, instead of just using the MS provided API. However, I am currently building a prototype, and I wanted to use the built-in controls, which leaves me no choice but use the Membership API.

Turned out it's quite simple, you don't even have to create an aspect:

Dim section As MembershipSection = _
   ConfigurationManager.GetSection("system.web/membership")

section.Providers.Add(New ProviderSettings("InMemory", _
    "Tests.Membership.InMemoryMembershipProvider"))

section.DefaultProvider = "InMemory"

Here I add my custom provider to the configuration and set it as default.

Of course, this simplicity shouldn't fool you that this is a good idea in general.