I looked at my Avistar that is mostly retired.
My smaller planes that use a nylon bolt to hold the wing in place use a dowel on the leading edge.
That is going to be difficult to do on the Avistar. Re-shape the wing, beef up and then modify the front of the wing seat.
Adding a brace from side to side for the wing bolt and blind nut would be easy enough. You would need to add wood to the inside of the wing ad Bruce suggested. One to add to the structural strength, two to keep it from trying to slide side to side. Piece of wood made exactly the same size as the fuse opening and placed so as to keep it centered without the bolt in place. Need to beef the fuse side also. Nothing to hold that wing in place but the rubber bands and the fact the wings servo tray and arms will not let is slide to far as it is.
Unless you move the one aileron servo and install two I do not think there is room for two bolts, at best very tight.

For the leading edge fuselage wing seat it will take some work and you need to keep the airfoil correct. Ho2zoo has the right idea. Need to recess the bolt head, heads and design for that. I know it can be done. I just might do it myself. Might not, I like it stock and its clean design. But for me building is a large part of the fun. I landed that plane once with the wing barely hanging on, busted rubber bands hanging all over. One reason it just sits. I have not bought rubber bands this season and I will not fly it with old rubber bands. Mostly I have other planes I would rather fly.
I flew it past its design to do that damage to the new rubber bands. My fault.

Seems to me four bolts would be easy enough but more of a pain to install the wings than rubber bands. 3 not as bad but still a pain to line up and it would still need something to keep the wing in place. That wing would be prone to sliding unlike planes designed for dowels and a single bolt.
Whatever you do you need to watch the weight, center of gravity and airfoil shape.

If your friend does it and it works.....post a picture.
Good Luck
Michael