Splicing on deck

[Tedder]

So far, all my double-braid splicing has been at my bench, but now I need to work on deck with the fittings in place (travelers, down-haul, out-haul)--it is too much trouble to remove them. At the bench, I can hold a shackle or eye and bring the splice home pulling against a loop securely fastened, but in this case, the shackle is securely fastened while the rope is free (except for a loop tied to keep the core from wandering).
What is the theory to proceed in this case? I am sure you riggers do splices in this condition all the time, so what is the technique?
Thanks!