It's still up in the air if that cover will be sold via reservation; Evening Call hasn't said one way or the other as far as I know, but it's currently leaning towards a C81 exclusive. The last small event (no where near on the scale of Comiket) was My Wife, and they had they had a few "event exclusives" there; Alice and the Valkyrie chick. But they had preorders open for about a week, so I'm hoping they might do the same thing here. But given the scale of Comiket, it's usually unlikely.
However, Evening Call recently had to change their material in a rush, so it's possible they may not have the stock they'd like for C81, and decide to open a web reservation because of it... or I guess it could go the other way, since they'll have a limited stock and want to save it for the event. Either way, as far as I know, it hasn't been decided for sure. Generally speaking, at least with normal preorder covers, they'll usually have a few extra and you'll be able to order them on the site a few weeks after the release day.
Edit:
Just checked Evening Call's site, and they posted this in the comment section. It's a translation, so it should be taken with a grain of salt, but it looking more like an event exclusive unfortunately. They said they may decide if they want to sell after the event, but it isn't guaranteed.
Changes in fabric production time will be very tight, so, it must be held the first exhibition distribute minutes, pre-order will probably not be able to do. Then, the situation on the day of sale, will consider whether there is a mail order after the event. Thank you.