I agree that an SBC is not the best reference case to validate everything against, however I bet that for very most server software, including PMS, the share of SBCs it is installed on is massively increasing, since at least the newer generation are greatly capable of doing nearly everything (great GPU acceleration support included, more and more even in mainline Linux) in small office or home environments. Not a killer argument but something that could be kept in mind 
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The 512 MiB default for tmpfs, hence /tmp on the other hand is not related to the underlying board.
I was not sure if there is any practical difference between intro scan transcoding and “regular” playback tanscoding so that there is an argument to use different cache directories. In case, there surely was an argument to put the transcoding cache by default into /var/lib/ instead of TMPDIR, and the same argument would apply for intro scan transcoding as well?
Basically I vote for keeping TMPDIR untouched and use it as it is/was for small tmp/runtime files while, for simplicity, the already existing (+setting in web UI) separate TranscoderTempDirectory for both, playback transcoding and intro scan transcoding, following Orko.
I bet it is already implemented greatly but it came into my mind that for intro scanning of course only a short time of the video needs to be transcoded to catch the intro, however either this is already practice or technically not as easy as my limited insight makes me imagine 
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