Custom Security Plug-ins? Possible?

Is it possible to build a custom data layer that we could use to present a login page to hit our internal tables before granting someone access to our data? We investigated the idea of using Excel for SSAS access, but struggled with a user-mapping-security solution to limit what people can see based on internal data-driven permissions.  We'd love to be able to use MS solutions, but this is definitely a sticking point for us as we have 30k users who are not in our domain. We can't easily map them to domain users because it's possible that each one of them could have different security rights. (Not likely, but there could be a rather large subset of rights.)

We got to see one possible way to handle passing users through security tables when processing security, but weren't sure how we could pass them through a custom front-end to give them a login before letting them into our server.

I'm not sure if I'm explaining this correctly, but if there are some tips on how to write a custom login module so we can access SSAS over HTTP using some form of mapping, that could go a long way towards solving some of these issues.

Thanks.

New in SSAS 2008 is a full suite of AMO objects in .NET allowing programmatic control of almost every featuer in SSAS. I have a lot of clients taking advantage of this to extend SSAS and provide additional customization to automation, security, etc..

One of many articles on the subjects here: http://technet.microsoft.com/en-us/library/bb500153.aspx