500L, Optimum Barrel Length?

Yes, if used as a key fob, shorter is better. My current (but built over 25 years ago) 5-shot 500L Blackhawk is 3.75". Aesthetics, seems 5.5" is popular. Max to belt carry comfortably, probably 7.5".

But for ballistics? As an example, take the 327F Single Seven, generally looks like velocity decreases with the longer and intuitive 7.5 over the 5.5. Most of the 500L reload data reviewed (like Hodgdon) uses an 8" test barrel.

Should the next 500L be a 7.5 or 5.5, performance considered? Or is barrel length performance negligible from 5.5 to 7.5 for the 500L? And thus, is powder burn rate, primer, and bullet selection (limited) the greater factors?

I'll throw out a slightly different thought.
Since you are looking at a custom,, and you already have a short one,, why not go 6-1/2"?? It's a bit longer than the 5.5,, and shorter than the 7.5,, so packing would be more comfortable, and the performance would likely balance quite well.
I think the one I tried had a 5.5,, and I felt it would benefit from a little longer tube.

Looking seriously at the new BFR Bisley in 500L short cylinder, currently only offered with 8" barrel (MRI calls it a 7.5") but 5.5 (6" per Ruger standards) should be available later.

Not crazy about white micarta or the sharp un-beveled edges of the grip panels. Also looks like a step backwards with the hammer spur versus the older SBH style. Only a couple of files, cut off wheel and a TIG from being nice?

Try taking a look at the BFR custom shop. You can get pretty much any configuration you want and have the confidence that your gun is a hand build variety as opposed to a standard production gun.

Everything I have read indicates the custom shop guns, aside from any custom features, are a step above the production guns.