I think a recap is in order, if only to save myself having to re-read the whole thread to remind myself where I'm at.
1st Experimental Project: I now have a way to charge the car that's lightweight and can be squeezed under the bonnet so I can lock the car up without needing to supervise. The downside is @6A max, it's a slow process (though no slower and far more desirable than having to previously heave the lead-acid out of the car and put it on my mains charger).
This alone was the main driving force of all this as I didn't want to have to buy two new batteries that will inevitably suffer accelerated and premature sulfation in my ownership.
2nd Experimental Project: The option to jump-start from the lithium pack still awaits testing as it's no longer a priority since removing the tracker from the C3 and given the success of the first project. Nonetheless, it's worth adding to my toolbox and will offer further benefits, I feel.
1st Bonus Experimental Project: I can also recharge the lithium pack off the car's alternator though more tests are needed to see if I can completely do away with the need for a dedicated in-car lithium charger.
2nd Bonus Experimental Project: Ony just occured to me the other night while I sat waiting for the windscreen to defrost.

9000 interwebs goes to the first person to guess what I've since ordered?