Yes, they supply 5VSB which is usually 3A or less, should be listed on the PSU label. You can power anything you want with it but keep in mind that a high load can wear out components in that subcircuit (especially the capacitor(s) ) because the PSU fan won't be running.
If you have that slight a load, you're better off just using PS-On to turn main PSU circuits on and running the light load off the main 5V rail.
You'll lose a little efficiency but gain PSU lifespan and if you really want high efficiency, use a cell phone USB charger instead for this low 5V load. However heat can again become an issue, for a high % /day runtime you will get longer lifespan by drilling several vent holes in it.