Stanchion repair problem

[tikiboy]

Hello everyone..
A friend of mine has this problem on his houseboat that he is "redoing".
Problem is that the previous owner bolted down some homemade stanchions without a way to put a nut on the backside of the stanchion plate....there is no way to reach the underside of the plate..
There is no way to get to the bolt's thread end ..so I guess they are screwed into the deck....he hasn't taken them out yet.
He may have to unscrew the bolts (they have hex heads and I guess they're lag bolts) and stuff some fiberglass filler into the holes and rescrew them down...
Doesn't sound like that would be strong and last....
Now, the metal stanchion / railing has rusted off (not stainless)....and fallen off the boat entirely....leaving the stanchion plates.
So he's left with a stanchion pad that is bolted to the deck.....I guess without a backing plate or nut because there is no way to get to one..
Would you recommend just filing the holes and screwing the new plates and stanchions down. like before...
I wish they made a stainless bolt that kind of opened up like a toggle bolt...and would collapse when turned CCW to take the bolt out for future maintenance....
But the toggle would have to grab onto what's behind there....
This is very hard to explain....
Any of you had this prob..?

[Sailmonkey]

You mean something like this?

https://www.amazon.com/TOGGLER-SNAPT.../dp/B0051IB1X6

[Benz]

It would be helpful to know what the houseboat's made of, and what the plates are lagged into. They may not be lags at all--perhaps there's a threaded insert potted inside. It'll be easier to give advice if the plates are removed and we see what's under.