I am thinking that perhaps the server that the electrum client uses could be used for something like this?
Certainly, electrum servers are easy to use, but unfortunately not (yet) very stable. You would also need to code the actual creation of a transaction (although electrum code is easy to read so that shouldn't be a big problem).
It looks like the electrum server depends on bitcoind and bitcoin-abe. Looking at the database that bitcoin-abe creates I think it is just the thing for what I need to check balances and the sendrawtransaction feature of bitcoind will help me send.
If I figure this out I will post links to the source code.