One possible solution (if none forthcoming from elsewhere) is to modify an existing SimBrief a/c profile to match the Arrow III essentials. Then just use that profile. At present I have been using the Commanche, but I think the C172 is more suitable. Also, remember to edit the aircraft type when filing the flight plan.
The problem is that it no longer seems possible to edit the cruise TAS when doing this. Thus ETE's, ETA's and fuel consumption are never correct.