Well I piked out.
RichFaces 3.0.1 + Ajax4Jsf 1.1.1 included the major fix to RF data tables that we needed (http://jira.jboss.com/jira/browse/RF-37), without too many side-effects, so we're going with that option.
We'll consider a major Seam 2 / JSF 1.2 / RF 3.1.1 refactor when we have time on the next project, and hope some of the niggles will be worked out by then.
Thanks anyway.
David