I know that Nazism has co-opted a number of eastern ideas and symbols, and the number 88 is auspicious in Buddhism, but is there any evidence this number held any significance to the original Nazi's themselves?
Cursory online browsing suggests 88 signifies HH meaning 'Heil Hitler' but I was unable to find any evidence to support this other than common acceptance.
It didn't have any significance. 88 stands for Heil Hitler, Heil Hitler wasn't a banned slogan in Nazi Germany. It started in the Neonazi scene (I'm not sure about the date but I am very sure that it wasn't directly after the war) as a code as it was banned by the "Wiederbetätigungsgesetz" (the nazism ban law).
So it only started to get used as a way to circumvent anti-nazi laws after the war.
As a source, Austrian DÖW (Documentation archive of Austrian resistance) works also on modern right wing extremism and they say that too. Online it is only in German.