I'm an old High Guard player, so the idea of spinal mounts is nothing I have problems with.
I'd say that they'd be the "next stage" technology in BT naval architecture. BT physics doesn't tend to go exotic like meson beams; while the maths is dodgy, the materials science is very much "yes, but more" of current physics. Plus the state of cybertechnology appears less advanced than current 'real world' - mainly in terms of miniaturisation.
Perhaps the Star League might have gotten there, had it not had that minor unpleasantness. Perhaps something based on HPG/Jump technology might have come along, rather than exotic particle weapons. But the current game future doesn't look great for that sort of R&D being prioritised.
Would make a great campaign hook. The Lyrans are working on a Capital Gauss spinal mount throwing doorknobs the size of dropships, the Capellans on WarShip Stealth, the Kuritans on a spinal Uber PPC mount that can carve the coordinator's face on a moon. the Feddies on the Rotary Capital Autocannon spinal mount, etc ;)