Yes. This is not supported, but it can be done.
What you need is a pool of connection pools. Sounds silly, but using a tool like proxool, it can be done.
There is one unsupported implementation of this in the iBATIS CVS repository. You can browse it here if you want, or check it out using a cvs client by following the instructions here.
The people at proxool are working on a DataSource implementation that does this (look here for more on it), and we have discussed the possibility of them using the one in our CVS repository as a starting point.
Hopefully, there will be a simpler solution for this soon.