I know where you're coming from - I regularly kitbash engineering vehicles, and intend to get some LTAVs 3D-printed as amphibious APCs.
But in most cases, these units don't have any real effect in a game with 10-second turns. Like combat engineers, it takes more than 10 seconds - more than 10 minutes - to dig field fortifications, to set up a bridging unit, to lay demolition munitions. So there's a powerful reason not to develop extra crunch rules for absolute fringe game situations. And therefore less desire to stat up many combat or support vehicles for these fringe situations.
Alternatively, go off-brand. "Yes, this is the FWLM. That's not a Davion Prometheus, it's a Leaguer Covenant-class bridgelayer!" Or just take a Scorpion, replace turret with a CF40 bridge, and use it as an objective - "must make it off the map so it can be used elsewhere". Track turret hits against the bridge."
And sorry - while the 'metal donut' idea fits in the game rules, have you ever considered why modern militaries don't do something like that? Laying a bridge takes minutes - I'm sure you can find a video on YouTube. Are you happy to have the donut-deliverer sitting immobile for 12 turns, potentially under enemy fire, just so a light vehicle can become a stationary target? Better to use combat engineers "pre-game" to dig in berms, lay minefields, etc.
W.