I don't have experience with the Berkeley XML DB. I have used the BerkeleyDB 'Java Edition' (a different codebase and on-disk format) for multi-GB spill-to-disk queues and maps/sets.

The BerkeleyDB license is roughly: free to include in open-source projects, pay if you want to redistribute as part of a closed-source product. So like with the GPL, using strictly inside your company might not be considered 'distribution'. But also, if you are selling software licenses their bundling rates might not be bad compared to your unit price. Of course, do your own license analysis before business use.