I have this summer fitted a secondary mainsheet in the cockpit. It runs from a stainless steel U bolt fitting mounted through the sole in front of the binnacle to a new fitting near the end of the boom. I used a 4:1 block and I left the original main in situ, set to protect the shrouds in the event of any mishap. The 4:1 purchase works like a dream and I can now trim the main from the steering position (one handed). The area under the cockpit sole was reinforced with 50mm x 20 mm thick steel plate running the full width of the binacle area and was glassed in.
The whole thing took me a couple of hours and approximately £150. The two ends of the new main sheet fiddles are clipped via SS closed carabeiner type shackles which enable removal in less than 30 seconds. I can provide pictures if anyone is interested in replicating the process.
I have overcome the problem with the traveller, I now have a small 4:1 block between the mainsheet bottom block and the deck fixing using 5mm dyneema. This provides lateral adjustment by extending the bottom sheet it moves the point of purchase from a central point to a lateral point (visualise a V with the bottom being fixed but the two top points moving wider apart the longer the sheet is) the main sheet when hauled in tight will never reach the central point so you will always be in a spill condition on the main. Before anyone asks, you don’t need it when sailing in light airs, you can use the coach roof traveller to haul the main to windward as there will be less pressure and more time.