For what it is worth, here is my take on the subject of bumpers on the S30.
The chassis is 100% sheet steel, formed into a monocoque to support the suspension loads imparted while driving, while also carrying the engine, drivetrain, interior (seats, dash, wiring, spare, etc.) glass and the bodywork that doesn't contribute to the structural integrity of the car.
The early bumpers were flimsy at best, only included in the design to satisfy the safety regulations of the era. The addition of the 5mph bumpers changed the chassis little, primarily with the addition of reinforcement where the shock absorbing cylinders were mounted to the ends of "frame".
And then the bumper only needed to protect the car's occupants in a 5mph collision.
Look at any of these cars that has been involved in a collision even at relatively low speeds, such as 25 to 30mph. They crumple up like an empty beer can. And the bumpers are often lost in the wadded up sheet metal.
So to your question abut an inner bumper to support the aftermarket airdam/bumper. Absolutely, a close fitting tubular inner support would work great to reduce the damage done by inconsiderate jackwads who view cars as an appliance and lack any regard for awesome classic sports cars. There will still be damaged, albeit mostly cosmetic, a simple repair.
Hit it harder and as Charles points out, the impact loads are likely to transfer a great deal of the energy into the framerail and inner structures that share the loads in that area (remember, monocoque).
I've owned quite a few cars and pickups over the years that were worthy of my constant attention, washing, cleaning, polishing, maintaining, repairing, and constantly repaying my efforts with the ego boost when some other like minder person says, "Hey, nice ride!"
Its a labor of love, but I try not to love anything that cannot love me back.
But I have always been annoyed by the turds who lack the capacity to show respect to another person's vehicle and not cause damage to it.
So I always carefully choose a parking space. Protect the passenger side with the planter at the end of an aisle. Park far enough out that there is less likelihood of someone parking within ten spaces (remember, the inconsiderate ones are also the lazy ones, so they always clamor for the front, nose in their stoopit phon checking FacePlant).
If I don't have a passenger I crowd the right side of the stall.
On the road, I drive both offensively and defensively. Watch traffic close an far away, easing up in intersections, glancing right and left in anticipation of one of the aforementioned dipsticks with their face in their stoopit phon, not focused on the task of driving. (cross my fingers and knock on the fake wood on my Z steering wheel) I haven't been in an accident that was my fault in over 40 years, and only two where the other driver was at fault (and had I been more focused on driving might have avoided).
It is your car so the final say is yours, so do what your best judgement dictates.
Then do your best to avoid the distracted dipsticks.