When looking into possibly using +5v light strips in my Speaker Light Kits...I hadn't thought about the (SAM) USB port for power. Though I had heard of LED lights using USB for power in other applications, it didn't cross my mind. Hearing about the specs you have listed, it wouldn't have done me any good as the +5v light kits I have now need 10x that much current.
I see the Pinball Life Accessory Power Supply for SPIKE and SPIKE 2 games offers the same 500ma of current at +5v that you mention the UBS port should be able to provide. So maybe that is a reasonable limit for some +5v mods out there. I just can't think of many +5v mods......I think I actually started a thread at one time asking what different mods used +5v. Yeah, here it is....a real hot topic there:
https://pinside.com/pinball/forum/topic/what-mods-use-5v
Doug (SpeakerLightKits.com)