what's this crane capacity ?

500# is about 2 really big guys weight, grab a couple and see what happens...not over water or the dock ..lol...

At the time of those boats (1987), an Avon Rib at 10' with a small outboard of less than 15hp was "state of the art". I imagine that this deck mount crane would not be appropriate for anything over 250 lbs. regardless of the winch capacity.

A standpipe from the sundeck floor to hardtop would improve things, but then would be a daily obstacle on one of your main living spaces......

That arm won't support more than about 200# at the most.
The other issue you need to look into is the max weight you can put on the hard top. I would not want to be sitting under that thing with a 500# dinghy on it. Check for a name plate up on the rear of the bridge. If its not there give the mfg. a call. These set ups were designed for 8-9' dinghy's with maybe a 9.9 on the back. Not the 500# consoled version your thinking of.

I do see the standpipe in the picture now, but I would agree that we have about a 200 - 250lb set-up. The manufacturer is no longer in business, so the risk is all on the OP.......

For clarification's sake, when I refer to the name plate on the bridge, I'm talking about the aft deck "roof" not the crane. There is usually a load limit plate for what can be safely placed on the top of these .
PacBlue as you stated back in '87 no one had a dinghy that weighed more than what you described. This type of setup is just not meant for the new type of "high end" dinghy's we enjoy today.
One other point I'm concerned about is even if it would hold the weight, I'd be very concerned about adding that much weight that high up, you really change the COG. In bad weather that could be a problem.

There are still options within those weight limitations. I've both known and read many people who still believe tenders or dinghy's should all be rowed. Many with those thoughts are loopers. Then there are small portable outboards. A small Zodiac is under 100 pounds. a small outboard under 50 pounds. Oars even less. A lot depends on use. But something along these lines is what your boat and lift were designed for. For just short calm water use these dinghies work great.

The OP has posted this same query on another site. He apparently doesn't plan on putting a dinghy on the top of the aft deck. (Silly us) He would like to use the crane to hoist a motorcycle onto the aftdeck? Not sure how ? But I'm pretty sure I read it right.

Then he might want to go to a scooter or a bicycle would be even safer.

We had something similar on our Huckins and it could only lift approx 250 lbs