Is a 14-32 acceptable?
http://harriscyclery.net/product/shimano-hg-50-7-speed-cassette-14-32-quotfquot-728.htm

I don't believe you can get a 34T cog in 7 speed, so you'd have to use a 34T off an 8 or 9 speed.

The slightly thinner cog won't be a problem since it's on the end of the "stack".
The problem basically is that it appears the only 7 speed cassette with a 14T top cog is the one listed above.
That means you'd be spending a lot of $ to gain 2 teeth on ONE cog.