It sounds like you have pretty much exhausted the options. Maybe just fit the new rubber to the Zipp and move it to another model of pump, e.g., Lezyne?
It looks like the Silica is the same thing and its rubber replacement is £12 (bastards)
Twitter suggest Hirame and Tekno but I'm sure I used to have these style heads and they didn't fit - maybe people are using shorter valved tubes than me? I'll have to revisit.
It sounds like you have pretty much exhausted the options. Maybe just fit the new rubber to the Zipp and move it to another model of pump, e.g., Lezyne?