Rich,
I agree with you on Option 1, however it would be possible to deploy a different version of the assembly to the GAC and then have your BTS apps reference that new assembly through Assembly Redirection in the btsntsvc.config file. You would need to ensure that your interface remains consistant however.
Option 2 seems the most sensible given that you can update the web-service without affecting the rest of you integration layer, however bear in mind the latency going over the network introduces and as you rightly pointed out, you are exposing business-logic and implementing SOA where it isn’t really needed.
Option 3 also seems sensible, given that the deployed BRE policies can be accessed by your entire BTS tier and C# codebase if necessary. You get versioning ‘out-of the-box’, but bear in mind that the latest policy version takes precedence. Furthermore, ‘non-techies’ can update the business rules if necessary.
In my opinion, I would stay away from option 2 as it introduces too many additional overheads, so its a toss up between options 1 & 3 – if you want non-techies to update and deploy your business rules go with BRE, otherwise go with assemblies and redirection.
On a final note, be thankful you’re in an organisation where there is healthy debate, I’m currently over-ruled by C# developers who want to throw out BizTalk and do everything as Windows Services. Sigh.
HTH, Nick.