What do you mean by boot loop? What exactly is happening? If it shows the red low battery symbol then goes to the Apple logo and repeats the problem is that the iPad wasn't properly shut down and the new battery has too little charge to start it up so it goes through that loop. A different charger capable of delivering more current may allow you to charge enough to boot then shut down properly. Otherwise you can try plugging in the charger, unplug as soon as the Apple logo appears, and when the home/login screen comes on very very quickly plug the charger back in. iOS should shut the iPad down and allow it to charge. The power/home button reboot thing you mentioned is another way to address this, but realize it takes like a gazillion tries to get it to add 1% to the battery, and if you take a break you're back to square one.
One other solution, the one that I'd use in my shop as I don't want techs sitting around holding buttons all day :) , is to open the iPad again, remove the battery, and charge the battery directly by applying ~4.2VDC directly to the battery + and - terminals using a DC power supply.