I feel impossible to make a numerical opinion about that situation. My best answer is only: try and observe. Sparks may damage the fibres but probably not the oven.
I expect the microwaves to reach the metal with nearly full intensity. The SiC foam should absorbs them slightly: enough to get hot and catch an interesting portion of the available microwave power, but no significant shielding. Unless the foam part conducts better than some 400Ω.
The composition of the metal won't change usefully the risk of sparking. Only the shape does. Very round edges at the tube's ends would improve. Closed ends are better than an open tube. Ceramic coating there can improve too by reducing the field through permittivity and by withstanding higher fields. This needs a significant thickness (until the field has spread into more area), and a good design isn't trivial, but less efficient tinkering is conceivable.
The composition of the metal does change the ability to withstand heat. Aluminium is just bad, usual and special stainless steel far better, then you have nickel alloys for gas turbines, and beyond that only exotic materials. Corrosion, creep, and sublimation limit the capability, where materials beyond nickel superalloys can withstand corrosion and sublimation but creep badly.
Magnetic permeability lets absorb microwaves over an even shallower depth ("skin effect"). I expect no consequence here: in all cases, heat diffusion is what will limit the skin temperature.
The gas composition can help. Already drying the air limits sparking. High pressure increases the breakdown field almost linearly. Some gases are better: SF6 best, CF4 excellent, dry air good. Deep vacuum would be excellent too.