What PSC Says
I asked Pacific Seacraft the same question, noting that mid-80's Ericson 38s had an inner forestay option. Mark Kohlmann at support@pacificseacraft.com replied that "The only reinforcement typically required is a backing plate on the underside of the deck. You, will, however have to use your own judgement here. The inner stay does not handle a significant load relative to the perimeter standing rigging." I would take this to mean the design loads are more like halyard loads (2000 lbs) versus forestay loads (5000 lbs). One way to scope out the loads is to ask your sailmaker for the wire size used in a wire-luff stormsail of the size you want. Then 40% of the break load can be your design criterion. Also consider going with a wire-luff stormsail, rather than a fixed stay. That way you wont have to deal with stowing the inner stay on deck. Note a babystay typically has a different function from an inner stay, namely foreward support of the mast.