I couldn't determine how long you were going to leave that sulfate/citric solution, so yeah - hot water may work. I'm of the understanding the K-meta and citric is more of a long term storage solution, not a cleaning/rinsing solution. I'm very paranoid about my barrel storage and go out of my way to avoid chemicals for the reasons mentioned in that wiki: I don't want to impart any long-lasting sulfurous effects. I don't even use tap water for rinsing/storing, either - it's all DI. I've read that even the TDS in tap water (mainly chloride, i think) can mess with the barrel flavors, too. You're on the right path by dechlorinating your tap water. I don't deal with any of the chemicals, so I'm not the one to suggest how to use them.
I rinse my barrels after dumping (if not filling them immediately) with cold water to "wash" them. I'll then store wet, using only hot (200 F) DI water: boil it up and transfer, roll it around, then just bung it up. That sterilizes it (not really, more like pasteurization) to the best of my ability, to prevent growth during storage and keeps it from going dry - avoiding a ton of work to swell it back up if it takes a long time to come back to it to fill it. Every now and again to prevent something growing in there, i'll repeat the process with fresh water. Then the day before fill day, I'll do as you suggested: boil up some water, transfer to the barrel and swirl to sterilize (not really) and let it sit hot for the day.