I like the sound of the steampunk mod. If machines are more "meta-scale" than "this block does this" then it'd be great. And if you're converting any power to any other power, give it a hefty penalty. Like, if you could burn coal in IC2 for 100 energy, or you could burn coal in BC for 100 energy, then converting IC2 energy to BC energy should only give 50 for one coal. This makes it possible, but highly restrictive. Also RP energy is ridiculously abundant, watch what happens with one or two solar panels over a brief time. Converting RP power to any other energy should incur massive loss to compensate for how much energy is produced.
Zeppelin airships were very buggy last I checked (several versions ago) and would often crash SSP and didn't accommodate modded blocks, like, at all. So I'd built an IC2 mobile quarry, using mining machines and battery boxes, but when I started moving, the ship would get weird and like, lose IC2 energy or shift wires or just crash.
Plus it's smooth movement, but it feels lame to me. You can move a mountain, for the cost of a few iron. RP frames are slower, but they're very costs and seem more realistic. Mobile bases shouldn't be cheap, this is a lot of potential for use, and moving at 20 blocks per second using the most common ore available... Well, I'd rather have my heavily engineered frameship.
Worth noting that the cannons in the Balkan's mod have a rather small explosion. You could probably make your ship impervious by using plating. Obsidian would definitely be protective, but the explosion is pretty light, so even something like cobble might protect. Although IC2's reinforced blocks would likely be ideal. In true arena play though, there would likely have to be rules about what materials to use for the hull, the structure, and the drive core. For instance, there could be rules that the hull can only be reinforced with stone/marble/basalt or cosemetic materials like wool and gem panels, the central core structure could be reinforced with something tougher, so that the cannons couldn't destroy them, and the ship would always have things like walking paths and redstone lines. Then the drive core could be done up in obsidian or reinforced stone. The engines are expensive, making them always survive would be very polite.
So, your average frameship fight would have two ships firing cannons at each other to crack the hull. Once the hull is open, then players can move in and plant TNT to blast the frame and the internal machines, and finally expose the indestructible drive core, at which point the fight is "won".
Also worth noting that in the most recent vanilla snapshot, mobs can move through portals.