i believe you could put a piezo in with very little/no issue.
This afternoon I took a bit of time to Google "piezo tachometer" and found several commercial units available. So the concept is not new and readily accomplished.
no high pressure tube to injectors.
As I understand the "piezo tachometers" they are based on the fuel injection pressure and not the exhaust. From post 13 Maybe the OP will be able to adapt the concept to exhaust pressure.I've also seen reverence to an "acoustic tachometer" which is a phone app for deriving engine speed from the sounds. https://play.google.com/store/apps/details?id=com.javiery.rpmgauge&hl=enI am still very unclear why there is no access to a rotating piece of the engine. I'd consider drilling a hole through the flywheel bell housing.How do the tachometers which the vendor sells for the high prices work? What signal do they use for rpm.