Artificially increase TransactionID?

Greetings:

I am interested in increasing the PostgreSQL TransactionID, as part  
of testing a (yet another) replication system that I am working on.  
(http://bruce.codehaus.org/ for the interested). I would like to test  
what happens when the transactionID crosses 2^31 and when it wraps  
around at 2^32-1. Other than running "select now()" 2^31 times, is  
there a practical way to raise the transactionID by large amounts?

I need to test against Pg 8.1.X and 8.2.X, and probably 8.3 while Im  
at it.

Many thanks in advance.....Bob.