Kev / Pol,
Pol, the one by the bridge I have glanced at a few times while crossing, as Kev says, I'm pretty sure the road access will be non existant. Kev, I think I've seen the slip you are talking about in Alloa (pics on Geograph) but not had a chance to get down there to investigate it.
As for heading up the river, I've been over the A9 & railway bridges by stirling Rowing Club more times than I care to count, so seen it at all states of the tide. I reckon that if you either had a SIB or timed it to within about 15 mins either side of a spring tide and cranked your engine up a notch or three you could get under the A9 & ye olde bridges (Rly bridge seems to have a bit of water under it) and get to the tidal limit (just visible form the M9 bridge) Only problem is if you get it wrong, it's a very long wait to get back out.......