I'm using my 64GB iPad & CCK for this same purpose.
to delete all the photos at once, connect your iPad to your Mac via USB cable, then launch the Image Capture app on the Mac (app is included with OS X). you can do a Select All, then press the delete button in Image Capture (or of course delete just selected photos)
p.s. of course you can also delete imported photos within the Photos app on the iPad itself, but it is more tedious if you have a lot of photos. As you've notices, unfortunately there isn't a Select All option on the iPad, but you can select multiple photos for deleting.
e.g. you can go to the All Imported album, then click on the icon at top right (the one that lets you email, copy or delete a photo). Tap and hold a photo until you see a blue checkmark, then if you drag your finger over other photos without releasing, you can select multiple photos. then after selecting the desired photos, tap on the Delete button. as I said, this is kinda tedious for more than a handful of photos, but at least you can delete small batches of imported photos directly on the iPad this way.