So, I have become involved in a POS defence vs Wingspan. They are sieging out a 4 meatbod corp of Germans who are rather newb at the whole thing. So really all that Wingspan has going for it is numbers and a keenness to spend hours and hours of their evenings plinking away inefficiently at structures.
This really drives home the fact that you need to prepare ahead of time for these eventualities. Whilst it is fun to buy and own all the toys and stuff your hangars full of ships, if you don't have your defence strategy as equally well-stocked, you may as well SD your ships now, and save an alliance like Wingspan the effort.
The key is preparation.
Besides having a well prepared POS (you know, one that deters mostly all attacks anyway) you have to be prepared for the siege. This means having spare POS guns for when your original set are incapped, as you can anchor them and online them even at a reinforced POS.
The problem is you can't store them in your CHA. You instead need a strategic reserve of iterons or whatever, loaded with POS guns and ammo, sitting in your SMA, ready to go. That way you can extract the hauler from the SMA, launch and anchor the guns, and voila, instant new POS defence.
Get on it, small corp people, or you too will suffer much strife.