An interesting approach to use db links. However as pointed out earlier in this thread, starting in wM 6.5, an adapter service call has a input pipleline parameter called $connectName, which you can use on the fly in your flow service to specify the connection to be used.
Also note, since its the adapter connections which manage db pools, selecting any connection on the fly will not have any adverse effect on it.
Meanwhile, in 6.1, we have designed a dynamicMap Java service as follows:
a) We pre-defined 4 adapter services pointing to 4 databases.
b) The dynamicMap java service uses a PSUtilities package service called doInvoke(), which allows it to call the appropriate adapter service on the fly.