I can't think of any issue as to why it wouldn't work if the RA is JCA compliant. Unfortunately, I don't know of any open source Siebel solution and I am not sure how Oracle handles distribution of the Siebel software (evaluation etc).
I know that there is a Siebel JCA adapter being that WebSphere/Portlet stuff makes use of it. If you are able to try/test this and run into issues let me know. I would be interested in getting a standard configuration together for the examples.