Using web bundles won't tie you in to dm Server as they are being standardised in the OSGi Alliance and the web container used by dm Server is being developed as the Reference Implementation of that standard.
A web bundle is a way of implementing a servlet and you could write one which would return a SOAP payload in order to implement a SOAP/HTTP web service. (For other web service transports, such as AMQP, you'd need a non-servlet model and web bundles wouldn't then be appropriate.)
You should also be able to use Spring Web Services to help you implement a SOAP/HTTP web service, but please report back if you see any issues.
If you prefer to use CXF and Jetty on top of dm Server, then you might consider running just the dm Kernel so that you won't get dm Server's web container and associated Tomcat server. But that route would be unusual, so expect to do some trail blazing! Again, it would be interesting to hear how you get on if you go this way.
I would recommend that you try using web bundles and Spring Web Services. The tooling support in the SpringSource tools suite should help you and you are more likely to find help in these forums if you hit problems.