rowID|client|feed|refreshInterval|expirationInterval|expirationDate|refreshesInBackground|downloadsEnclosures|maxEnclosureSize|enclosureTypesString|unreadCount
...
82|1|81|-1.0|0.0|224917200.0|1|0|0.0||0
299|2|0|1800.0|1209600.0|224917200.0|0|0|8388608.0||0
301|3|0|604800.0|-1.0|214027200.0|1|0|8388608.0||0
...

which was produced by the command: select * from subscriptions;. Note the entries with rowID=299,301, which have feed=0. I believe these entries store the default settings for the corresponding client. So since row 299 has client=2, it corresponds to Mail.app. I verified this by changing the refresh interval in Mail.app, which changed the refreshInterval for row 299 from 1800.0 (i.e. 30 minutes) to the modified value. I wanted a refresh interval of 10 minutes, so I executed:

update subscriptions set refreshInterval=600 where client=2 and feed=0;

As the original poster mentioned, the proper way to do this is through the API. It's actually pretty easy to get and set the refreshInterval for a client with a few lines of code, but this is useful for those of us that want a quick hack.