Interesting. I've never been a fan of the principal of separated circuits unless your wanting to run different coolant/wetter etc. The leaking intercooler issue you mention adds another dimension to the design as well.
...........So when your WOT and your cooler is leaking- say MAP=>16psi - it pumps up your coolant tank/system and then I guess the cap releases at something less than this (even though it's not effectively boiling) - so you get residue/spray coming out everywhere. Wow!
I guess that's one way of confirming you have a badly leaking IC then.

I did have a link to the Beamer PS tank on ebay ~$20 but it's just dropped off my watch list.