Is the WC electronic so that she can change angles? Does she have a custom chair cushion? The biggest thing will be offloading in order to get it to heal. As you know, even slight changes every hour can help. Without offloading, wound care doesn't matter too much aside from staving off infection.
No matter what you put into the wound, some type of foam dressing is best as it can help keep at least some pressure off the area. Collagen powder isn't a bad idea. It can be used with silver gel, which would provide some antimicrobial effect and a little moisture if the wound is dry. This can be purchased over the counter. Make sure they change the dressing if she is incontinent or if it gets soiled.
When doing any sort of packing, like iodoform, just make sure it is loosely tucked in there and not tightly packed, otherwise the new tissue cannot fill in against the packing.