On a rig with in-line spreaders it's not advisable to but a multi part purchase directly into the backstay unless you also have a wire strop as a safety (ie set at the minimum tension setting). With swept spreaders it's unlikely the mast will fall down if the tackle fails or is accidentally let run free, but with inline spreaders the backstay is the only thing pulling back on the stick. This is probably why the split backstays are popular for adjustment.
I think hydraulics or mechanical 'screw' type adjusters are otherwise the way to go.
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)