A two-part epoxy resin would probably work, although it might add some 'shine' to the rock and it does yellow over time. I've also heard of folks doing a muriatic acid bath to neutralize carbonates in Seiryu stone.
I've done the acid wash, but it was to try to darken up elephant skin stone. My experience with the process is that it made the elephant stone whiter and not darker, lol. I've read numerous threads where muriatic acid baths will not prevent stones from leaching compounds into the water column.
I think I might experiment with a product called Zoopoxy. It's what those guys on the show "Tanked" supposedly use. Ever heard of it?