Shouldn't be a problem. I've had a bunch of bikes with both campy and shimano batteries in the seatube without holders. They are usually very easy to get out.

My battery actually got stuck pretty bad in the seat tube as the tube is a weird shape that easily wedges the battery in place. If you can, I would tape a string or something on the bottom of the battery so you can easily pull it out when you need to.

I just received by x68 and there appears to be a carbon fiber lip in the seat tube that is preventing me from installing my DI2 battery far enough down in the tube to adjust my seat height. I suspect this is where 2 different carbon parts were joined together.

Is this fixable? I think cutting my seatpost shorter and drilling new di2 holder holes will not get me the minimum depth required.