Proots,
Your BW-7
, I believe
, is a cone clutch gear box that is similar in design to a Kanzaki/Yanmar box and not similar to a typical Hurth/ZF box that uses clutch plates. I think the only engines they were used on were some Bukh's and a few Westerbeke models. The BW-6 I know is a cone.
They were built by a Spanish subsidiary of ZF called INDUSTRIAS SUBSIDIARIAS AVIACION SA but are not Hurth's..
With most cone clutches neutral is preferred and the manual states this. I, like you, would strongly question that "forward" will not damage this box.
Yanmar/Kanzaki go so far as to specifically prohibit locking it in any gear while sailing and suggest a shaft brake if locking is desired. Their boxes are also cone clutches. I suspect Westerbeke knows little about what will harm this box having sold so few of them compared to the Hurth/ZF's or Yanmar with their many thousands of cone clutch boxes being sold.
It was not until 2008 that Yanmar changed their advice on sailing position with cone clutches, and that is after perhaps selling 50X or more the number of cone clutch gears when compared to the JS's.
I would try and email someone at ZF and tell them what you have and get a better assessment than what Westerbeke, and their distributors, have given you.
P.S. That box does NOT use ATF so be careful with that. I have seen some folks put ATF in there and they were told to do this by their engine installer....