[plt-scheme] adding to custodian shutdown actions?

Hi,

Is there a way to add a custom cleanup action to those that occur when 
(custodian-shutdown-all) is called?  Specifically, I have a database 
connection that I was forgetting to close (I thought GC might close it, 
but I guess not).  I'm thinking this is the kind of thing that could be 
  managed by a custodian, but I don't see any procedure like 
`custodian-register-resource' or `thread-register-cleanup'.

Rob