Thanks for this thoughtful response shivaoutdoors. Effectively towing/floating a barrel has certain risks and dangers involved - but so does paddling a heavily overloaded sluggish kayak (or submarine). Like most things in life, the perfect solution probably does not exist.
Over the years have heard (true?) stories of cooking pans on heads and gear stored between the legs when trying to cram everything into a kayak.
Anyway, now realised the need to go back to the drawing board with all these suggestions - effectively the attachment system I proposed only works well on swallow (or flat) tail kayaks - and even then getting the balance just right is a bit fiddly. Will report back if I find a good solution.
To clarify how I would envisage working with a barrel:
Grade I-II - tow barrel
Grade III - free-float barrel - but following it like a sheepdog (exception headwall rapids)
Grade IV and over - portage barrel
Have paddled quite a number of classic wilderness rivers (with support) but more often in low water conditions. Certainly would not think of taking a barrel post-monsoon on the SK.
Hopefully someday on a wilderness river somewhere, I will be the one with a barrel parked-up on the bank whilst ripping into the perfect wilderness wave - smirking to myself as the heavily charged kayaks trundle past. In my dreams?
Thanks to everyone who has given feedback on this proposal.
Ian
Summary of Pro and Contra of Barrel system to date based on this forum (not in order of importance):
Pro
(1) the kayak can be kept light for better performance/safety in rapids;
(2) the kayak does not need to be repeatedly unpacked for stop
and play, portaging or overnighting;
(3) less likelyhood of hurting your back lifting overheavy kayaks;
(4) for longer trips buy a bigger barrel not a bigger kayak;
(5) allows for taking extra rations/gear for emergencies;
(6) you can take your river running kayak with you rather than a big boat - easier to fly-in;
(7) cheap - easily available compontents;
(

Gear split in 2 places (boat and barrel) - less risk of losing everything.
Contra
(1) risk of losing barrel;
(2) risk of hurting other river users (minimal in wilderness);
(3) added risk of having 'anything' attached to kayak on a river - automated and quick-release imperfect.
(4) not suitable on all rivers (eg continuous ww, technical, log infested etc.)
(5) towing set-up is fiddly to adjust and balance;
(6) drags more than a bigger boat;
(7) impaired stearing and acceleration (only applies on flat bits);
(

rolling impaired plus added risk of self-injury (clutter);
(9) looks stupid and inhibits aerial moves (joke! - please dont blow my karma any lower or I wont sleep tonight);
(10) if you should swim (on III - see above) you have 3 separate things to worry about - paddle, boat and barrel
Did I forget anything?