While I have no experience with RoR, we have taken a similar approach with Django as an interface, where we will use java-based services to complete large-volume processing transactions.
We define a database in the django model, develop a web interface with a django / jquery view, and, obviously, we get a free admin site.
On the java side, we run the hibernate reverse engineering task to create DB access classes for Java code. Where necessary, we use the JSON engine created by JSON on top of AMQP RPC to communicate between python and java components.
There are ups and downs in this approach. For us, since we did not use any large java frameworks, we provided us with access to a quick / lightweight web interface for our solution, as well as the simplicity of modeling our database using the Django model format (which I really like).
At the bottom, we came across some tricky things with reverse engineering hibernation configuration (which is difficult if you are doing something that is not a standard of the swamp, especially regarding some of the Django ManyToMany relationships).
I noticed that you already mention that you already have a Spring based architecture with a web tier. I am not an expert here, but this may present you with opportunities worth exploring, for example, groovy / grails or SpringMVC, which in version 3 claim that they are significantly simplified and faster to develop. It can give you the victories you need.
A compromise might be to take a look at some RoR-inspired Java tools such as Rife .
The bottom line is that you will need to make a call in light of what your own application requirements and team capabilities are. This worked for us, and I think the solution is right for us, but if your team is already equipped with Java and Spring skills, then Spring 3 might be the way to go (but ask the Spring expert about this :-))
Rob