This is what i would suggest...
When having foreplay as you tease her slowly introduce anal play. Start with simply teasing, working slowly (VERY slowly) up to occassionally inserting a well lubricated finger, then two, then start working towards the plugs and such. You should be able to tell her interest from the teasing and the finger play. Plus, if she does enjoy it the enjoyment might help her get past her inhibitions until she is asking for it to go futher!