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is there EVER a need for that antenna's output to be run through an FM trap filter?
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Yes. That long boom picks up a LOT of FM, especially when situated at or near a right angle to the boom's aiming direction.
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Does that possibility change if it's filtered through a standard UHF/VHF UVSJ combiner, with the vhf side capped?
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Yes, the UVSJ would be expected to attenuate everything, including FM, below the UHF port's cutoff frequency by a minimum of 20 dB. It would take an extraordinary level of FM signal to require a second stage of filtering.
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Or if the UHF antenna is pre-amplified?
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I've learned the hard way that it's usually better to keep FM out of your pre-amp unless you have a very robust amp that won't overload or produce spurious signals.
A spectrum analysis would be needed to pin down specifics, so you're usually left with following "best practices" if you want to side step those issues.