I think we might have been talking at cross-purposes here!
Chapter 8 about general "cost of capital" whereas when we talk about Solvency II we use the "cost of capital approach" to determine the risk margin.
So, in Chapter 8, this is a more general chapter and in this context, the cost of capital from holding reserves could be any capital that has to be set aside.
In Solvency II, we are determining the risk margin and so just apply the cost of capital to the non-hedgeable cashflows from the SCR calculation. We do this because this is how the SII regulations require it to be calculated!
More generally, you could allow for the cost of holding capital in a similar way. So, work out the capital (using any definition you decide, if we're using SII it could be SCR + BEL + RM as you suggest) required each year and then the "cost" of holding this is a proportion of the amount required.
Hope this clears up the confusion!
Sarah