Joe Breeze HiteRite altenative... ideas before I get hacking!
Many people do seat posts that  achieve this - have a search on google for "Dropper seat post", All of the current models are almost certainly more reliable and easier to use than a simple spring, and can be activated while riding. The fact you cannot find any like the HiteRite is probably the effect of what we now refer to as Darwinism - not bad in it's time, but things have moved on since 1984.