Thermionics don't really matter. Also, anything that would be applicable, is going to look very different from what you're imagining; consider for example alumina (or even more inert) ceramics, layered with electrodes, machined with cavities, and maybe a tungsten wire spans across one gap for heater, a mesh grid electrode on the next lyer, and plate on top. Connections are made with metallization, or metal enamel, and sealed with braze or glass frit; the completed assembly is sealed in a high vacuum furnace brazing operation. Reason being, the miniaturization is much greater, a necessity for any kind of technical mission -- but it's doubtful that anywhere near enough computational power would be practical with such a scheme. It could manage flight systems, some sensor processing and communication, but nothing like the high-resolution photos or video that are demanded today.
Not to mention power systems; even with 300V+ solar panels (or RTG stacks), you've basically got awful-efficiency thermionic switching converters, or mechanical switching -- straight up switching (relays), or [even] old[er]-school vibrator supplies, may well be worthwhile; not sure about arcing at high temperatures though, that might be questionable. Maybe high pressure Ar fill would do or something.
Hm, solar. Light level at the surface IIRC is a fraction of Earth's, maybe like a heavily cloudy day, or on Mars. The cloud layers are VERY thick! The spectrum is more-or-less visible white (a bit yellowed I think it is?) , meaning solar panels need a bandgap of 1-2eV to function. But if they're cooking at 500C, even a 2eV bandgap is pretty much useless. Kind of nuts to think about an RTG in such an environment, but the mechanisms all still work at high temperature AFAIK. Don't know what materials would be applicable. I'm sure PuO2 would still be fine as fuel, probably casing materials will have to improve, especially for chemical resistance.
Battery technology at least is available; Mg-Sb and Na-S chemistries could do at those temperatures.
Would certainly be fascinating if we mined an honest to goodness whole-ass asteroid, separated iridium from the metal fraction, and use that to build things. Dropping a few care packages worth down to the surface would easily pay for the mission, and such a strong and inert metal would survive fine in the atmosphere. I'm sure there are easier and cheaper ways to protect such a thing, but more to say it would be neat to do it that way some time.
But, needless to say, they'll probably use something like SiC or GaN, or even diamond if reasonable dopants can be found (perhaps the high temperature enables more options?), and make ICs out of that. Imagine a PPC CPU crafted of transparent crystal and running at 9V or thereabouts! It'll probably run slow (10s, maybe 100s MHz?), but have enough data width and memory to be usable.
It would be difficult to design tubes to handle the harsh environment of any launch and landing with multiple-G loads and wild temperature swings.
Long-solved problem, remarkably; one of many technologies that won the war, the proximity fuze. They put a homodyne receiver inside a
fucking artillery shell; it worked, devastatingly well! IIRC, the tubes were fairly standard design, subminiature glass, with extra reinforcement. Power was delivered by a battery whose electrolyte came from a vial (smashed by shock) which then got squished into electrodes by centripetal action.
Tim