The Thumper and Arrow IV systems weigh 15 tons each, before adding ammo. It's hard to get a simple modification kit that'll replace the AC-5 on a Vedette, even dropping the turret for an SPG configuration. You'd have to drop the engine rating and armour, at which point it becomes a little complicated IMHO.
OTOH, they do produce the Carriers - LRM, SRM, AC-2, etc. 34 tons of warload mean you could get a pair of Thumpers, or Arrow IVs, with 4 tons of ammo total. That to me is a useful idea, and a cheap way to upgrade your firepower. The weak armour & poor speed of the "Carrier" suddenly matters much less.
In terms of "artillery" - or at least fire support - I could see an "indirect fire package" with an LRM-15 packaged with a MG & half a ton of ammo, that could in theory replace AC-5s on Scorpions or Vedettes. But perhaps not quite what you're looking for.
