By the way, in the meantime I've managed to solve my problem.  Here's how:

• Used "pywallet --dumpwallet" on my backed-up wallet to get a list of private keys.
• Deleted the wallet and started Bitcoin-qt.  This created a fresh wallet.
• Wrote a shell script to iterate through all the old private keys and import them using "bitcoind importprivkey account rescan=false"
• Restarted Bitcoin-qt with rescan option

Step 3 was key: initially I tried importing also using pywallet.  However, Bitcoin-qt was then unable to read the wallet.  The solution was to use bitcoind itself to import the private keys!