I'm in a similar proverbial boat to Pol - V. busy round then so not sure if I will be free. If I am, I'm definetly up for it. Any idea of the planned date?
Regarding navigation, WRT ribs, Up to the railway bridge shouldn't be an issue. I have been plotting this cruise for a while, and reckon if you could arrive at the railway bridge 20 mins before a high spring tide there is a half a chance you might get through the old arched bridge and onward to the tidal limit. But as said, it's high risk WRT geting back in time, so you probably want to take the "I wuz 'ere" pic & turn round.
One other option is take your SIB dinghy, anchor the rib off the rowing club & do the last half mile in something you could "shoot the rapids" with in a worst case.
I reckoned last week as I drove over the M9 bridge at high tide you could have motored all the way to Doune on the plane!