Well, some companies use a dewatering process when working on a pool in high water table (drill and pump off water while working.) it jacks up the price for sure but you could ask.
In your shoes, I don't think I'd go the dry acid route -- it adds a lot of sulphate to the water which can cause problems if built up too high.
I've never used the delivery systems you found, but I suspect that may be your best option short of a "wet" or no drain acid wash. I don't know enough about proper care of plaster to give worthy advice on this topic and don't want to see you exacerbate the situation.
There is a poster called OnBalance who is very knowledgable about plaster...might be worth a pm to him?
Another avenue that's not likely especially cheap or quick in the DIY department (depending on whether in stock near you) would be to take a look at the Jacks Magic stain removal system, testing with their stain kit, and then using the recommended remover...I believe they have one that's more effective on copper. While at TFP we kinda eschew the risk of getting pool stored, Jacks co. is reasonably astute in that they're cognizant of stain testing methodology...their formulations are pretty targeted, etc. In my experience, their products and intel have been useful, though in terms of sequestrant, I also like ProTeam's metal magic.
Either way, you'll want some sequestrant in that water because if you lift those stains, the metal will redeposit if its not given something else to bind to...the MA doesn't dissolve them or anything.
You can also call Jacks Magic and ask for a technician...they're quite good about giving situational advice and matching appropriate product and procedures. Have your water parameters on hand when you call, eg your calcium level in particular.
Hope that helps give you some resources. Take a look at the sequestrant and stain remover lineup here and use the 1800 number :
Jack's Magic Products, Inc.