I think I've got there.... from a combination of most of your replies.
The main reason I'm wanting the 'pump circulating to dump heat' safety system to work is because a) I've got it, so may as well make some use of it; and b) I don't want to rely on just one safety system that kicks in at 95deg in a 350lt store; and c) If I can avoid washing the heat down the drain, so much the better!
Like I said, I think most of my questions have been clarified to one degree or another, both on here and via some external research. I've concluded that a fairly simple 3rd pipe to rad - entering at the top, opposite the return should work - no reason I can think of why it won't, and confirmed on here by Mysteryman.
This pipe to be 'normally closed', with a 3-port valve opening the pipe when the 90deg overheat stat on the store triggers the pump to do it's safety work. I need to add this valve to the system.
So, under normal day-to-day conditions (including when the solid fuel stove is not lit), I'll have a fully TRV'd system (plus I'll add an ABV because my new pump is not modulating as far as I can tell).
If the stove is lit, and the store gets to 90 deg, and I have electricity, the 1st safety system (already fitted to the store at the factory) will kick in - it will start the radiator pump (to all rads), and it will also bypass one TRV on a large rad (just in case all TRVs are closed). In effect this is like having a 'permanently' open dump rad - but only when conditions dictate that I need it open.
If I do not have electricity, the 1st safety system will not work at all - no pump, no diverter valve, no circulation - so it is irrelevant whether the port valve will work or not. This is where the 95deg power-free quencher coil (also fitted at the factory) will kick in and pour lots of hot water down the drain.
End result - I need to add a 3 port valve and a run of pipe to the top of the relevant rad. I'll keep the port valve near the store so I can test it periodically. I also need to add an ABV (but not for the reasons I'd originally suggested).
I think that covers it....
Many thanks again to you all for your input. I'll post back in due course to let you know if it works ok - unless it kills me