Adding velcro in would be very easy, as access is straight at the tip with the hole you will open up . The A15 has no permanent opening just velcro. Velcro only opens when you have a lot of water and it usually self closes, so I can't see a disadvantage to it. Having no permanent opening the sand needs to be emptied manually every so often. It would be easy to make a permanent opening by just damaging some of the hook side of the velcro.
The bag is interesting. You could just stuff the kite straight into the bag and avoid the rolling. That is how paragliders pack their wings. I used to do it with kites, as was quick and avoided sand. I don't bring a bag now and a rolled kite is much quicker to setup as you can use the wind to unroll it.
Rolling anything bigger than 10-12m is quite hard. But if you want to try real hard try with no board or vest! Kicking legs just love to tangle in lines or bridles and then try not to drown. I highly recommend flotation in light wind and getting lines and bridles onto the bar quickly.