You can get endpin jack pre-amps, which you should be able to fit inside quite easily, though they don't have an adjustable volume control. The Graphtec Ghost pre-amp would be ideal, as it's designed for accepting both a magnetic and piezo input. It comes with a 'summing board' for summing individual Graphtec piezo saddle signals (such as Ghost saddles fitted to a Strat), but you can ignore that and just wire to the piezo input directly.
It's not particularly cheap but it is compact and should fit inside the control cavity (maybe with a touch of enlarging). You can use the basic model and add the pots and extra switches yourself (you can solder the connections to the header pin arrangement) http://bmusic.com.au/index.php?main_...ucts_id=3481or there is a more expensive kit with the switches and a piezo volume pot included (you use your own magnetic pickup volume and tone pots). http://bmusic.com.au/index.php?main_...oducts_id=3482
Or you might find something really cheap on eBay you can modify.