Would using some concealer powder work -- assuming the shedding isn`t too severe yet, you might get away with it.
Debbie sells a lace repair kit which is basically a light nylon thread and needle, also a length of lace and a glue..... if you were to try your idea, it might be worth getting that kit, if the area is relatively large. I`ve not heard of someone ` patching` a fullcap like that, but I guess it`s feasible, as long as you`re careful when cutting....that kit might be worth getting though, otherwise how will you join the edges.....alternatively, I reckon if you cut out a ` square` from an old topper, a little larger than the one you cut from your fullcap, then shave off the hair all the way around, just a FEW millimetres inside the edge of the lace, that would give you a ` bare` surface, so you could glue that to the lace on your system....you`d have like an ` island` of hair on the ` patch` with `bare` lace all around the edge, which would adhere to the ` inside` of your base, with the hair ` poking` out of the area you had cut away.....that way, you wouldn`t be trying to join edge-to-edge---it would be very hard to do that ..the lace would keep fraying, so you`d have to stitch them together `in` a few grids of lace, and that would leave a really lumpy seam. .hope that makes sense !
You`d be best to pin the system to a styrofoam head, to secure it properly, before attempting a cut, otherwise you`ll easily go all crooked, with your line....same with cutting out the ` patch` ..pin it down to prevent movement.
This is an example of why it`s best to always have a ` standby` system on hand.......sudden shedding can occur and leave you in this predicament.
Think I`d use ` Nair` to ` clean` / remove some hair around the edge of the patch..if you try shaving the hair ,you`ll probably slice through the lace. I guess, try making the ` patch` first..see how you get on with doing that, as there`s no harm done if you mess-up with that, before deciding whether or not to cut any part of your fullcap.
You could also consider just using tape to hold the patch onto the system.